2x - y = 10
[(x^2 + y^2) - (x - y)^2 + 2y(x - y)]/(4y)
= [(x^2 + y^2) - (x^2 - 2xy + y^2) + 2y(x - y)]/(4y)
= [2xy + 2y(x - y)]/(4y)
= [2y(2x - y)]/(4y)
= (2x - y)/2
= 5