The volume of a cylinder is pR²H where p = pi, R = radius, and H = height.

The question suggests that the heights of both cans are equal, so we don't have to worry about that.
Concentrate on the surface.
The surface of the smaller cylinder: pi x 3²
The surface of the bigger cylinder: pi x 5²

Code:
pi x 3²    243
-------- = -----
pi x 5²     Y

where Y is what we need...
I hope I don't have to tell you how to derive Y from that