The sum of 7 and the cube of a number.

7+x3

Step-by-step explanation:

Sum means to add the addends together. A number is a word form of a variable, and you are cubing that number which is multiplying that number by itself three times.

if you are saying purchasing 3 social media ads and one newspaper ad counts as one purchase then there are:

6 purchases

if you are asking how many of each then:

18 social media ads

6 newspaper ads