For a cylinder:
S.A. = 2πr2+2πrh in this formula, r is the radius and h is the height.
S.A. = (area of circles) + (area of tube part)
To solve question 1, you actually need to know what the radius is. Otherwise you have one equation with two unknowns.
The setup for question 2 is the same formula, except you'd plug in x for the height instead of h. Since the diameter is twice the radius, r is also equal to x. Just apply the formula above. You can cancel π out and things simplify to 400 = 4x2.
Hope this helps. If you need more help, let me know.