Hi Jenny,
"m" is the replacement for a number that you have to figure out.
I'll show you how to solve this with simpler numbers so you'll see how to do it.
5+11+7=19+m, m=?
When you add the first three numbers, you get 23. So 23 = 19 + m and you need to figure out what number m is. So what number do you add to 19 to get 23? You can subtract: 23  19 = 4. So m=4 in this example.
You have to be careful with your own problem, because the numbers are bigger, but the process is the same.
I hope this helps you.
Stephen La Rocque>
