Question 1021436
a)  If g(x) = x, then 2g(x) = 2x and then f(2g(x)) = f(2x) = (2x)^2 + 4 = 4x^2 + 4
b)  To find an inverse, put y in for f(x).  You switch x for y and solve for the new y.  Then put f^-1(x) in for the new y.  Like this
y = x^2 + 4
x = y^2 + 4
x - 4 = y^2
y = sqrt(x-4)
f^-1(x) = sqrt(x-4)
c)  g(x) = x
y = x
x = y
f^-1(x) = x 
No surprise.