How would I simplify this equation, where do I start?

It appears that all they want you to do is to distribute the negative sign through the second parentheses and combine like terms. It would turn into:

x + 3y - 5Z - x + 2y - 6Z

Combine all the x's, all the y's and all the z's, and write the expression:

5Y - 11Z

Isn't that cool the way that the opposite signs on the x's make them disappear?

This is not an equation - there's no equal sign, right? We can't actually solve to get values for the variables. Is that what was confusing you?