College Math 105 issues...tutor center closed!

Ok, here is my question. Its on my homework and my Professor won't help us with this one, but I just don't get it!

(r+s)^2-16

Now what I was thinking was taking the square of the entire thing, that way eliminating the square and making the -16 a -4, but my Professor keeps saying u=r+s so I should substitute it. My thought is, where did u come from????

the other one I was wondering about was this:

-x(x^2+1)-3x^2(4x-9)

as an answer I got: -11x^3-27x^2-x but I don't know if that's right.

If you can help me at all, please reply. Thanks

Since you include no instructions, we have no idea what is to be done with the posted expression. Since we were not privy to the conversation you had with your instructor, we have little chance of guessing what he might have meant. Sorry!

Please reply with clarification. For instance, you posted this to the (post-calculus) "Differential Equations" forum, but I see nothing more than an algebraic (that is, pre-calculus) expression. It might help if you could at least explain how the posted expression relates to the study of differential equations. Including the section title (from your book) and/or recent topics of study (along with the omitted instructions) would probably be helpful. :idea:

Thank you!

Eliz.