∵x²-y²=4
∴(x+y)(x-y)=4
∵x+y=8
∴x-y=4÷8
∴x-y=0.5
∴3x-3y
=3(x-y)
=3×0.5
=1.5
请点击下面的【选为满意回答】按钮.