How do i solve this? (inverse variation)

Write an equation in point-slope form for the line through the given point that has the given slope (-2,-7); m=-3/2

Two customers went to a post office. first customer paid $12 for 14 and 5 envelopes the second paid 24.80 for 10 postcards and 15 envelopes. what is cost of each envelope?

Write an inequality and solve each problem. for exercises 11 and 12, interpret the solution.