3x + 2y = m + 1 ①
4x + 3y = m - 1 ②
7×① - 5×② 得到：
7(3x + 2y) - 5(4x + 3y) = 7(m + 1) - 5(m - 1)
21x+14y-20x-15y=7m+7-5m+5
x - y = 2m +12
x