# GED Math Extra Prep: Functions

Function problems on the GED Math test can often be solved using substitution. In the following practice questions, you’re first asked to find the value of a function *f* (*x*), and then the value of *x*.

## Practice Questions

*Both questions are based on the following function.*

*f *(*x*) = *x*^{2} – 4*x*

- What is the value of the function when
*x*= –4?

**A.**–8

**B.**0

**C.**16

**D.**32 - For what value of
*x*does*f*(*x*) = 12?

**A.**96

**B.**2 and –6

**C.**6 and –2

**D.**0 and 6

## Answers and Explanations

- The correct answer is D.
Rewrite the function by substituting in

*x*= –4:*f*(–4) = (–4)^{2}– 4(–4)= 16 + 16

= 32

Hence, Choice (D) is the correct answer.

- The correct answer is C.
Substitute

*f*(*x*) = 12 into the equation:12 =

*x*^{2}– 4*x*Subtract 12 from both sides to set the equation equal to zero:

0 =

*x*^{2}– 4*x*– 12Then factor:

(

*x*– 6)(*x*+ 2) = 0Set each set of parentheses equal to zero and solve for

*x.*You get*x*= 6 and*x*= –2. Hence, Choice (C) is the correct answer.