Factor 18x² + 39x + 21

Here we will show you how to factor the quadratic function 18x² + 39x + 21 using the box method. In other words, we will show you how to factor 18x squared plus 39x plus 21 (18x^2 + 39x + 21) 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 18x² + 39x + 21, like this:

a = 18
b = 39
c = 21

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

3	18x	21
3x	18x²	21x
	6x	7

We put 18x² (a) in the bottom left square and 21 (c) in the top right square, like this:

3	18x	21
3x	18x²	21x
	6x	7

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 18 times 21 (a × c), and add together to equal 39 (b).

More specifically, 18 times 21 is 378. Therefore, we need to find the two numbers that multiply to equal 378, and add to equal 39.

? × ? = 378
? + ? = 39

After looking at this problem, we can see that the two numbers that multiply together to equal 378, and add together to equal 39, are 18 and 21, as illustrated here:

18 × 21 = 378
18 + 21 = 39

Now, we can fill in the last two squares in our box with 18x and 21x. Place 18x in the upper left square, and place 21x in the lower right square.

3	18x	21
3x	18x²	21x
	6x	7

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 18x and 21. The greatest common factor of 18x and 21 is 3. Therefore, we write 3 to the left of the top row. You can see it here in the color green:

3	18x	21
3x	18x²	21x
	6x	7

Next, let’s look at the bottom row. We have the terms 18x² and 21x. The greatest common factor of 18x² and 21x is 3x. Therefore, we write 3x to the left of the bottom row. You can see it here in the color blue:

3	18x	21
3x	18x²	21x
	6x	7

To find the values below the table, we first divide 18x² by 3x (labeled in blue). This gives us 6x.

18x² ÷ 3x = 6x

You can see this value colored in orange below:

3	18x	21
3x	18x²	21x
	6x	7

Next, we divide 21x by 3x (labeled in blue). This gives us 7.

21x ÷ 3x = 7

You can see this value colored in purple below:

3	18x	21
3x	18x²	21x
	6x	7

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 18x² + 39x + 21. 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:

(3x + 3)(6x + 7)

That’s it! Now you know how to factor the equation 18x² + 39x + 21.

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