Julia wrote 14 letters to friends each month for y months in a row. write an expression to show how many total letters she wrote.

Let y represent number of months in a row and
let S represents sum of letters that have been sent in y number of months

This simply means that we can write equation:
14*y = S

For any number of months we express it in equation above and we get total number of letters she wrote.

The coefficient, 5 represents the length in relation to the problem.