Question

let f(x) =7x^2 -8
Find f(a^2)

Answer 1

substitute a^2 in x's place.
f(a^2)=7(a^2)^2-8
Simplify this.

Answer 2

$$ f(a^2) = 7(a^2)^2- 8 = 7a^4 - 8 $$