Factor -21x² - 25x + 100

Here we will show you how to factor the quadratic function -21x² - 25x + 100 using the box method. In other words, we will show you how to factor negative 21x squared minus 25x plus 100 (-21x^2 - 25x + 100) using the box method. It is a 5-step process:

Step 1: The standard form of a quadratic equation is ax² + bx + c. In this equation, a is negative. Therefore, we need to start by setting aside the negative (-). We do this by flipping the signs in the equation to get 21x² + 25x - 100. Now we can label the different parts of our equation, like this:

a = 21
b = 25
c = -100

Step 2: Next, we need to draw a box and divide it into four squares:

-5	-35x	-100
3x	21x²	60x
	7x	20

We put 21x² (a) in the bottom left square and -100 (c) in the top right square, like this:

-5	-35x	-100
3x	21x²	60x
	7x	20

Step 3: In order to fill in the other two squares, we need to do some math. We have to find two numbers that multiply together to give us the product of 21 times -100 (a × c), and add together to equal 25 (b).

More specifically, 21 times -100 is -2100. Therefore, we need to find the two numbers that multiply to equal -2100, and add to equal 25.

? × ? = -2100
? + ? = 25

After looking at this problem, we can see that the two numbers that multiply together to equal -2100, and add together to equal 25, are -35 and 60, as illustrated here:

-35 × 60 = -2100
-35 + 60 = 25

Now, we can fill in the last two squares in our box with -35x and 60x. Place -35x in the upper left square, and place 60x in the lower right square.

-5	-35x	-100
3x	21x²	60x
	7x	20

Step 4: Next, we need to find four final numbers to finish factoring our equation. We can see that our box is a 2 x 2 table made up of rows and columns.

Our goal is to find the numbers that, when multiplied together, result in the products in each square. We can do this by finding the greatest common factor in each row.

Let’s look at the top row. We have the terms -35x and -100. The greatest common factor of -35x and -100 is -5. Therefore, we write -5 to the left of the top row. You can see it here in the color green:

-5	-35x	-100
3x	21x²	60x
	7x	20

Next, let’s look at the bottom row. We have the terms 21x² and 60x. The greatest common factor of 21x² and 60x is 3x. Therefore, we write 3x to the left of the bottom row. You can see it here in the color blue:

-5	-35x	-100
3x	21x²	60x
	7x	20

To find the values below the table, we first divide 21x² by 3x (labeled in blue). This gives us 7x.

21x² ÷ 3x = 7x

You can see this value colored in orange below:

-5	-35x	-100
3x	21x²	60x
	7x	20

Next, we divide 60x by 3x (labeled in blue). This gives us 20.

60x ÷ 3x = 20

You can see this value colored in purple below:

-5	-35x	-100
3x	21x²	60x
	7x	20

Step 5: Now we have all of the information we need to finish factoring the equation. The values outside of the box are used to factor -21x² - 25x + 100. You simply put the values on the left in one set of parentheses, and the values below in another set of parentheses, to get:

(3x - 5)(7x + 20)

In our original quadratic equation, -21x² - 25x + 100, a is negative. Therefore, we need to add a negative (-) sign before our two sets of parentheses, like this:

-(3x - 5)(7x + 20)

That’s it! Now you know how to factor the equation -21x² - 25x + 100.

Factoring Quadratics
Go here if you want to learn how to factor another quadratic function.

Factor -21x² - 24x - 3
Here is the next quadratic function on our list that we have factored for you.