Let G(x) be a polynomial function whose derivative is G′(x)=kx2+6x, where k is a constant.
(i) Find the general form of G(x) in terms of k and the constant of integration C.
(ii) Given the two conditions G(1)=5 and G(2)=15, determine the values of k and C.
(iii) Write the complete expression for G(x) using the values of k and C obtained in part (ii).