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1. The streets of a city are laid out in a rectangular grid, that is 7 by 4. The 7 squares are the base and the 4 are the height.

A)How many routes begin with 3 north steps in a row?

The book has 8 and i can count 8 ways but i don't know how to get it.

2. The expansion of (3/x - x^3)^8 find

a) The constant term

b)The term containing x^12

For a) i did this:

C(8,r) x (3/x)^8-r x (-x^3)^r

I can't remember how to do it with a 3 as the top number. i can't add the exponents unless they have the same base so how do i do this.

For b is there an easy way to figure out which has x^12 which actually doing the whole expanding.

3. Use the binomial Theorem to determine the expansion of (a + b + c)^3

I have three terms how do i solve this.