Question 1005581
Hi there,
C varies directly with R and 
inversely with the square of S.
C = kR/S^2
21 = k17/1.5^2
21 = k17/2.25
21 x 2.25 = k17
k = 21 x 2.25/17
k = 2.78 (2 decimal places)
Equation:
C = 2.78 R/S^2
Hope this helps :-)