For the first one:
s = 2*pi*r*h
Solve for r like this:
s = 2*pi*r*h
r = s/2*pi*h , that is the last answer that is given as a choice, and it's correct.
For the second one:
c = 8*pi*a*b
Solve for a like this:
c = 8*pi*a*b
a = c/8*pi*b, that is the first answer that is given as a choice, and it's correct.
Basically, you need to divide both sides with everything that is next to the variable you are asked to be solved with.
Hope this helps!