Question

How do you know x when f(x)=2

Answer 1

Its always 2 does not matter what is defined to x

Answer 2

Say your function is
f(x) = 2x + 3
If f(x) is 2 & you want to find x, replace f(x) with 2:
2=2x+3
And solve for x:
-1=2x
x=-1/2
Hope this helps!