onlinetutor.com.au

We are trying to find out what number the letter x represents.

First, get all of the x's on one side of the equals sign (either the right or left side, it doesn't matter which).

For this equation, we will subtract 2x from both sides. Then we will have x's on the left side only. You must subtract the 2x from both sides of the equation, otherwise the sides will no longer be equal.

5x + 12 - 2x = 2x + 21 - 2x

3x + 12 = 21

Now, subtract 12 from both sides of the equation.

3x + 12 - 12 = 21 - 12

3x = 9

We now know that 3 x's are equal to 9. To find what one x equals, divide both sides of the equation by 3.

3x/3 = 9/3

x = 3

We have found that the letter x represents the number 3.

To check whether our answer is correct, substitute x = 3 into the original equation and see whether it works.

5x + 12 = 2x + 21
5(3) + 12 = 2(3) + 21
15 + 12 = 6 + 21
27 = 27, which is true so our answer x = 3 was correct!