"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.
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>