Hi there!
Let's solve this problem step by step!

To find f(3) we must substitute x = 3 into the formula.

Now we use PEMDAS (Parenthesis, exponents, multiply, divide, add, subtract) to find our answer.
Work out the exponents inside the parenthesis first.

And finally multiply.

Hence, the answer is D. 64.
~ Hope this helps you!
Answer;
Sean got 24 points
Katie got 31 points
Kevin got 8 points
Emily got 12 points
Sorry I cannot give proper explanation because of typo issues.
Hope that can help.
∵ ΔSRQ is a right triangle
and RT⊥SQ
∴ x² = ST * TQ
∴ x² = 9 *16 = 144
∴ x = √144 = 12