Here. The technique for these questions is to pick out the general term of the expansion using r (or something which is not x, y, a, b or n as a variable) and combine all the powers of x into a single power, so that the general term becomes a constant number multiplied by some power of x.

This is so that you can set the power of x to the required power (in this case the requires power is 0), because the different positive integer values of r (up to r = n) gives you all the different terms in the expansion (there are n + 1 terms altogether in the expansion).

Once we find the necessary value of r, we resubstitute the value of r back into the general expansion.