What is the asnwer worked out to m-9=-13

Thus is a linear equation, so you want to isolate m so you can compare the variable m to a constant.

You begin by getting rid of - 9 by adding 9 (its opposite) since a negative plus a positive can me zero. Then you add 9 to (-13) which is the same as 13-9 but with a negative value, so you get - 4.

The answer is - 4