One third the sum of 12 and a number can be written as 1/3(12+x). This equation can also be simplified to 4 + x/3, but sense the question doesn't ask you to do so, the answer most likely is 1/3(12+x).
When multiplying binomials, one takes the first binomial and multiply each term by the first term of the second binomial, and then you do the same with the second term of the second binomial, to obtain:
