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)(3x)^(9-k)(y)^k. For k=6, the coefficient is C(9,6)·3^3·1^6=2268. Remember that the numerical coefficient comes from the binomial coefficient and the constants attached to x and y, including any negative sign.