$$(y^{n+5})^2 : (((-y^2)^3)^2) = y^{2(n+5)} : ((-y^6)^2) = y^{2n+10} : y^{12} = y^{2n+10-12} = y^{2n-2}$$.
Ответ: $$y^{2n-2}$$