已知方程2x-3y=-7.求3(y+7)-6(x-y)的值

3(y+7)-6(x-y)
=3y+21-6x+6y
=9y-6x+21
=-6x+9y+21
=-3(2x-3y)+21

因为2x-3y=-7 所以-3(2x-3y)+21=-3*(-7)+21=42

3(y+7)-6(x-y)
=3y+21-6x+6y
=-3(2x-3y)+21
=-3*(-7)+21
=42

原式=3Y+21-6X+6Y
=21-6X+9Y
=21-3(2X-3Y)
=21-3*-7
=42

3(Y+7)-6(X-Y)=3Y+21-6X+6Y=21-3(2X-3Y)....
明白了吧

3(y+7)-6(x-y)=3y+21-6x+6y=9y-6x+21=3(3y-2x)+21=0

把原式拆开变成3y+21_6x+6y再变成_3(2x_ 3y)+21=42