1. f (x) = x2 - 21

f (5)

x=13

Step-by-step explanation:

if you will substitute the value of f (x) with 5, you will get the following equation:

5=2x-21

2x=26/:2

x=13

(I didn't know what you ment when you wrote x2, is it x square or 2X)

I really hope it was the second one, cause that's how I solved it.