Here is the explanation that was given.

"If f(x) = x^2 - 4, then what is f(f(1))?"

To find the answer, first find f(1). Then repeat the process to find

f(f(1)) --> f(1) = 12 - 4 = 1 - 4 = -3 --> f(f(1)) = (-3)2 - 4 = 9 - 4 = 5 --> 4

a) -3
b) 5
c) 8
d) 15
e) 21

I don't understand how to isolate f. It looks like f has to equal somthing then that somthing has to be squared ie. f(1) = f, f * f = f^2.