=>(x+3)(y+7)=(y+5)(x+5)
x(y+7)+3(y+7)=y(x+5)+5(x+5)
xy+7x+3y+21=xy+5y+5x+25
(xy-xy)+(7x-5x)=(5y-3y)+(25-21)
2x=2y+4
2x-2y=4
2(x-y)=4
x-y=4/2
x-y=2
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