题目计算:(1)3ab−a2−(2ab+3a2)(1)3ab-a^{2}-(2ab+3a^{2})(1)3ab−a2−(2ab+3a2)；(2)2(x2−xy+2y2)−3(x2−y2)(2)2(x^{2}-xy+2y^{2})-3(x^{2}-y^{2})(2)2(x2−xy+2y2)−3(x2−y2),其中x=2x=2x=2,y=−1y=-1y=−1.知识点：整式化简求值章节：数与式 / 整式
