Factor 3x² + 8x

Factor 3x² + 8x - 51

Here we will show you how to factor the quadratic function 3x² + 8x - 51 using the box method. In other words, we will show you how to factor 3x squared plus 8x minus 51 (3x^2 + 8x - 51) using the box method. It is a 5-step process:

Step 1: The standard form of a quadratic equation is ax² + bx + c. We start by labeling the different parts of our equation 3x² + 8x - 51, like this:

a = 3
b = 8
c = -51

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

-3	-9x	-51
x	3x²	17x
	3x	17

We put 3x² (a) in the bottom left square and -51 (c) in the top right square, like this:

-3	-9x	-51
x	3x²	17x
	3x	17

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 3 times -51 (a × c), and add together to equal 8 (b).

More specifically, 3 times -51 is -153. Therefore, we need to find the two numbers that multiply to equal -153, and add to equal 8.

? × ? = -153
? + ? = 8

After looking at this problem, we can see that the two numbers that multiply together to equal -153, and add together to equal 8, are -9 and 17, as illustrated here:

-9 × 17 = -153
-9 + 17 = 8

Now, we can fill in the last two squares in our box with -9x and 17x. Place -9x in the upper left square, and place 17x in the lower right square.

-3	-9x	-51
x	3x²	17x
	3x	17

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 -9x and -51. The greatest common factor of -9x and -51 is -3. Therefore, we write -3 to the left of the top row. You can see it here in the color green:

-3	-9x	-51
x	3x²	17x
	3x	17

Next, let’s look at the bottom row. We have the terms 3x² and 17x. The greatest common factor of 3x² and 17x is x. Therefore, we write x to the left of the bottom row. You can see it here in the color blue:

-3	-9x	-51
x	3x²	17x
	3x	17

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

3x² ÷ x = 3x

You can see this value colored in orange below:

-3	-9x	-51
x	3x²	17x
	3x	17

Next, we divide 17x by x (labeled in blue). This gives us 17.

17x ÷ x = 17

You can see this value colored in purple below:

-3	-9x	-51
x	3x²	17x
	3x	17

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 3x² + 8x - 51. You simply put the values on the left in one set of parentheses, and the values below in another set of parentheses, to get the answer:

(x - 3)(3x + 17)

That’s it! Now you know how to factor the equation 3x² + 8x - 51.

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