Simplifying rational expressions 4

Simplifying rational expression where each expression is a trinomial or difference of squares multiplied by a constant, a variable or both.

![simplifying rational expressions 4](https://f.cloud.github.com/assets/219302/798701/5c943f42-ed66-11e2-9b7e-e2f3ca0ec47e.png)

Accepts as many possible valid answers as I could think of, e.g.

7y(y−3), 7y * (y-3), (y-3)7y, 7y(-3+y), -7y(-y+3), 7y^2 - 21y, -21y + 7y^2

Wouldn't accept y(7y - 21): answers must be fully factored or not factored. Will be useful to make dividing_polynomials_by_binomials_2.html more forgiving and fix things like #50734