Case 1: Suppose we are given expressions like 3abc and 7abc and asked to compute their sum. If this is the case we should not worry much. Because adding like expressions with plus sign is as easier as adding positive numbers. Therefore, in this case add the numerals and suffix the common symbolic part which in this case happens to be abc. The sum will be, therefore,
3abc + 7abc = 10abc
Case 2: It is possible that all the like terms given may have minus (-) sign also. In this case also, we add the numerical coefficients and suffix the symbolic part to this numerical along with the minus sign. That is, if we are required to add -ab, -2ab and -2ab, we take the sum of numerical coefficients without their respective signs which will be 1 + 2 + 2 = 5. To this value, we suffix the common symbolic part (ab) along with the minus sign. That will be -5ab.