H(x) = √(1-x2)
We want H(x) = f(g(x))
g(x) is the "inside" function and f(x) is the "outside" function
For √(1-x2), the "inside" function is 1-x2 and the "outside" is the square root function.
So, if f(x) = √x and g(x) = 1-x2, then f(g(x)) = f(1-x2) = √(1-x2) = H(x)