求13.14.16题解,要算式,

13.把x.y带入那两个式子,则2a+b=2.2b+a=7
a=-1.b=4,所以（a+b）（a-b）=3x(-5)=-15
14.因为他们互为相反数,所以他们和等于0,则3x+2y-1=0,2x+y-3=0
x=5.y=-7
16.根据两个方程得x=2.y=-1,所以4-k=1
k=3