Please help me with this, i have no idea how to do this

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

Simplify the expression. -2/5(10+15m-20n)

Ab and bc form a right angle at point b. if a= (-3,-1) and b= (4,4) what is the equation of bc?