Actually Welcome to the Concept of the Mathematics, this is called as function inside another function, just we have replace the condition of x here,
1)g(f(4)) = here, f(4) = 4-7= -3
so now, g(-3) = (-3)^2 =9
g(f(4)) = 9
2) f(g(4)) = 16-7 = 9
f(g(4))= 9
3) g(f(-1)) = 64
4) f(g(-1)) = -6
done.