p(x) + q(x) will be in W if and only if (a+d)(b + e)(c + f) = 0. W contains all polynomials of second degree such that the product of the coefficients is equal to zero. Similiary, for kp(x) to be in W, we must have (ka)(kb)(kc) = 0. You have to check this before you can go on and say that p(x) + q(x) and kp(x) are in W.

Hope this helps!