a(a+b)(a-b)-a(a+b)^2
=a(a^2-b^2)-a(a^2+2ab+b^2)
=a^3-ab^2-a^3-2a^2b-ab^2
=-2a^2b-2ab^2
=-2ab(a+b)
=-2*(-5/2)*(-2)
=-10