I do not know how to do (a) part only, thanks for your teaching.?

set C = a r^3 + b r^2
r=1, 200 = a + b ...(1)
r=2, 1120 = 8 a + 4 b ...(2)
(2)-(1)*4
320 = 4 a ---> a = 80 ---> b = 120
C = 80 r^3 + 120 r^2 ......Ans