Like terms...
Well step 1 is to know what a term is. A term, in regards to math, is basically the things between any operation sign (+, -, *, /). Every time you see one of those signs in a problem, you're beginning a new term.
Step 2 is to locate the different terms in this problem. Remember, everything you see up until an operation sign is part of the term. Your term list, for this problem, is:
7a
4b
-3a
-2b
Got it? Cool. Now, we can find the like terms. a like term is a term whose variable and exponents are the same as another like term. Since none of these have exponents, do you see any terms that have matching variables ( which is the letter following the coefficient, which is the number)? Yep! In fact, there are two sets!
First set of like terms: 7a and -3a
Second set of like terms: 4b and -2b
Now that we have our like terms, we have to combine them! Ignore the variable for now... In the first set of like terms, what is 7-3? 4! For the second set, what is 4-2? 2! Since the first set goes with the variable "a", and the second set goes with the variable "b", you should get, as your final answer, 4a + 2b.