(f ◦ g)(x) is the composition of f(x) with g(x), equivalently
(f ◦ g)(x) = f(g(x))
To get f(g(x)), replace every instance of x in the definition of f(x) with g(x):
f(x) = -x + 7 ⇒ f(g(x)) = f(√(x - 3)) = -√(x - 3) + 7
