in my workbook it says give the opposite of each expression and the problems are:

-10t+21

-x+2z

there are alot more but i thought if someone explained a couple of these to me maybe id understand the rest. idk just please help me!!!

If by "opposite" they mean "negative", the answers are

10 t -21 and x - 2z.

Just put a minus sign in front of the expression they give you, and remember that two minus signs make a plus.