In an expression, there are terms, which are a number(s) that are combined by multiplication but separated by addition (if you see subtraction it's just adding the negative version of the number). In this case, there are 5 terms. A term can have a variable or it can be a constant. A constant is just a number. We can combine constants. For example, in this case, we can combine 22 and -8 to get 14. (just adding). If it has a variable, we can only combine two terms if they have the same variable, so 5m and -4m can be combined to get m. We replace the terms we have combined with the combined terms so we have m + 7n + 14.
