You need to represent the surface area as a function of the radius (r) of the base which means instead of having a number for S.A. you will have some number times r.
The volume of a cylinder is as follows:
v = (area of base) * (height)
v = (πr2) * (height)
20 = πr2h
now solve for height in terms of r (treat r as a number, not a variable)
h=20/πr2
Surface area is:
SA= 2(area of base) + (circumference of base * height)
SA = 2(πr2) + 2πrh
then plug h in from earlier
SA = 2(πr
2) + 40πr/πr
2
SA = 2(πr2) + 40/r
The main thing to keep in mind is that you just treat r like an ordinary number not a variable