I don’t understand the function

Kelly is a salesperson at a shoe store, where she must sell a pre-set number of pairs of shoes each month. at the end of each work day the number of pairs of shoes that she has left to sell that month is given by the equation s=300-15x , where s is the number of pair of shoes kelly still needs to sell and x is the number of days she has worked that month. what is the meaning of the number 300 in this equation

Ihonestly have no clue if it's each or none.

Which of the following values in the set below will make the equation 5x + 6 = 6 true? (only input the number.) {0, 1, 2, 3, 4}