[(x+2y)²-(x+y)(3x-y)-5y²] ÷2x
=[x²+4xy+4y²-(3x²+2xy-y²)-5y²] ÷2x
=[x²+4xy+4y²-3x²-2xy+y²-5y²] ÷2x
=[x²-3x²+4xy-2xy+4y²+y²-5y²] ÷2x
=[-2x²+2xy]
=2x(y-x)÷2x
=y-x