Freshman Algebra One help; due tomorrow

Ok here are some problems I need help with. Please dont give me the answer just a little help and some simple ways to figure it out!! here are the problems:

Write an algebraic expression for each verbal exprestion
15 decreased by twice a number

91 more than the square of a number

Evaluate each expression
11 with an exponent of 2

exelent condition books are sold for $2.50, fair condition books are sold for $0.50. Write an expression for the cost of buying e (exelent condition books) and f (fair condition books)

what are the factors in the algebraic expression 3xy?

*sorry I just feel so stupid and my parents cant help and my math teacher is a crappy teacher.

Well, for the first two, you need to start by replacing "a number" with "x." Does that give you some place to start?

Just use a calculator to raise 11 to the 2nd power. ( Which is 11 squared, btw. )

As for the story problem, start with the basics:

e=2.5
f=0.5

The number of books you are buying is x.

I'm pretty good at math, if I do say so myself, but it's hard to type the expressions, sometimes.

THANK YOU!