Both are D
First one:
f(g(x)) =f(x^2+3) then you x^2+3 is the x you are going to introduce in the function f
f(x^2+3)= sqrroot(x^2+3-3)= sqrroot (x^2) =x
Second:
you need to leave x alone
y = 3x^2 +1
y-1 = 3x^2
(y-1)/3 = x^2
sqrroot ((y-1)/3) = x
sqrroot ((x-1)/3) = h^-1(x)