Like terms are the terms having the same variables and being raised to the same index. A coefficient is the appended number in front of the variable; if no number is shown then we assume it to be 1.
For example and are like terms.
Unlike terms are the terms not having the same variables and or are raised to a different index.
For example, 4 and are unlike terms.
We can add like terms by adding their coefficients, we cannot add the unlike terms.
Now we are given the expression , so and is being added with .