SAT Math Question of the Day

Q: If (4 - x)(4 + x) = -9, then what is a possible answer for x?

1. 1
2. 3
3. 5
4. 7
5. 9

UNANSWERED.

The correct answer is C.

A:

One way to do this problem is to just plug in the possible answers and see which one fits the equation.

Let's try choice A, where x is 1,
(4 - x)(4 + x) = -9
(4 - 1)(4 + 1) = -9
(3)(5) = -9
15 = -9
Nope, choice A doesn't work. Onto choice B, x is 3 ...
(4 - x)(4 + x) = -9
(4 - 3)(4 + 3) = -9
(1)(7) = -9
7 = -9
Still not a match. Let's try choice C, where x is 5 ...
(4 - x)(4 + x) = -9
(4 - 5)(4 + 5) = -9
(-1)(9) = -9
-9 = -9
IT WORKS!
So the answer is x is 5 or choice C.

Another way to solve it is algebraically:
(4 - x)(4 + x) = -9
(x - 4)(x + 4) = 9
x² - 16 = 9
x² = 25
x = 5 or -5
So, C is the correct answer.