Ew. Composition of functions. Ok, f(g(x)) is what that fancy notation means. So, you would take g(x) & substitute that into wherever x is in f(x).
So we get something crazy like
f(x)= 4 - (.5x - 2)^2
Now, we foil out the squared quantity:
f(x)= 4 - (.25x^2 - x - x + 4)
Can you believe they call that simplification?
f(x)= 4 - .25x^2 + 2x -4
Almost there...
f(x)= -.25x^2 + 2x
Hope this helps!