Question 16Let $f:\mathbb R\to\mathbb R$ be a function such that, for any $x,y\in\mathbb R$, $$(x-y)f(x+y)-(x+y)f(x-y)=(6x^2y+2y^3)(x^2-y^2).$$ Suppose $f(1)=-999$. Determine the value of $f(10)$.