(-5x - y)^2 - 10x(-7x + y) =

(-5x - y)(-5x - y) - 10x(-7x + y) =
(25x^2 + 10xy + y^2) - (-70x^2 + 10xy) =
25x^2 + 10xy + y^2 + 70x^2 - 10xy =
95x^2 + y^2