In order to combine the like terms, you look at the constants and the variables. A variable has a letter in it. When looking at this expression, you find that there are two variables, 4t and -t. You then simplify these variables, getting 3t because it is 4t - t. Now, there is a two left, and since that ISN'T a variable, and is a constant, we can't do anything with it because we cannot combine it with anything else in the expression. Your guess was partially right ;)
Round each number to the nearest 10. 33 rounds to 30 and 89 rounds to 90, thus in your head it's easier to mentally add 30 + 90 for purposes of estimation.
