When you perform the synthetic division, remember the coefficients of x^3 and x. Both are 0. That will lead to the correct answer. The final factor has to be = to 0. It will be a+b+c.
The other way to solve this is:
If x+1 is a factor of the polynomial p(x), x=-1 and p(-1) = 0 according to the remainder theorem.
Set p(-1) = 0. Then solve for a+b+c


a+b+c = 0