Help pls :) for my test

What is the common difference in the following arithmetic sequence 7,3,-1,-5

Your friend uses c=50p to find the total cost, c, for the people, p, entering a local amusement park is the cost of admission proportional to the amount of people entering the amusement park? explain.

Find an bif a = {3, 6, 9, 12) and b = {2, 4, 6, 8, 10).