2的x次方=4的(y+1)次方 => 2的x次方=2的2(y+1)次方 => x=2(y+1)
27的y次方=3的(x-1)次方 => 3的3y次方=3的(x-1)次方 => 3y=x-1
x=4,y=1
x-y=3