what is -18 over 21 simplified to

First, think.

What numbers go into 18 and 21 (ignore the negative for now)?

Well, for 21 only 3 and 7 do.   

Now what numbers go into 18? 2, 3, 6, and 9

What do they have in common?

They both can be divided by 3

Now simplify!

 

-18/21

Simplify by 3

-6/7

And there's your answer