Use the binomial theorem and match the power of y to decide the value of k. Once k is known, the power of x is automatically n-k. The general term is C(9,k)(x)^(9-k)(y)^k. For k=2, the coefficient is C(9,2)·1^7·1^2=36. Remember that the numerical coefficient comes from the binomial coefficient and the constants attached to x and y, including any negative sign.