How do I work 9g-rm-9m+rg

Do you have to factorise?

Group what you see are similar in any two terms.

For example, if you had ab + because + cd, you could see that the first two have be in common, or the last two have c in common. Two different ways are:

b(a+c) + cd --or-- ab + c(b+d)

Try doing the same for your question.