to me your solution looks OK.
I'll do the problem and you can check your homework:
The volume of the cuboid is calculated by:
V = x² * h . Thus h = V / x². With your problem h = 5000/x²
The surface consists of 2 squares and 4 rectangles with length = height of the cuboid and width = side of the square. Therefore you get:
A = 2*x² + 4*x*h = 2*x² + 4*x*5000/x²
You can simplify the last summand by cancelling out one x. So you get:
A = 2*x² + 4*5000/x = 2*x² + 20000/x
And that's exactly what you've got. Congratulations!