Maths problem,how to do , thanks?

V = kr^2
r = 2cm ==> V = 27cm^3
∴ k = V/r^2 = 27/2^2 = 6.75(cm)

V = 168.75cm^3
  ==>  r^2 = 168.75/6.75 = 25 = 5^2
∴ r = 5cm