Can someone please help me on this problem I don’t really get it and it’s due really soon

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

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

Complete the solution of the equation. find the value of y when x equals to 1 9x-5y=29