Explanation:
I guess the 3.14 is to be used for pi (and not r, which is what we need to find out).
the volume of a cylinder is ground area × height.
the ground area is the area of a circle, which is pi×r².
so, we have
pi×r²×height = 115 cm³
3.14×r²×7.7 = 115 cm³
3.14×r² = 14.93506494...
r² = 4.756390107...
r = 2.18091497... cm