(cy^3)(9y^d) = 18y^6

Coefficient c on the left has to be 2 so that when we 
multiply it by 9 we'll get the coefficient 18 on the
right side. So we have: 

(2y^3)(9y^d) = 18y^6

Exponent d has to be 3 so that when we add the exponents 
3 and d of y on the left side, we'll get the exponent of
y on the right side, which is 6.  So we have:

(2y^3)(9y^3) = 18y^6

Edwin