The function x(a)= sqrt a/6 can be used to determine the side length, x, of a cube given the surface area of the cube, a. the function v(x)= x^3 can be used to find the volume of a cube. how can these functions be combined

It can be achieved by substituting the formula for length i.e x(A) = A/6 into the formula for Volume.
in the diagram( please mind my writing)
the x(A) is substituted into V(x) .
Only thing left is substitution of the values .
hope this helps .