Since 50 feet = 9 revolutions, then 50 feet = 9 x Circumference. Hence the Circumference = 50/9
C = pi x Diameter, so substitue and solve for D.
50/9 = pi x D
50/9pi = D
Not sure what, if any, approximation you are supposed to use for pi. But, you can plug that number in to get the approximate diameter.
Hope that helps!