Somebody please help!

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

This question awards 100 ! i really don’t want to fail i will also mark you !

(c) the diagram shows the first three shapes of a matchsticks arrangement.first shape = 3second shape = 5third shape = 7given m represents the number of matchstick used to arrange the n'th shape.(i) write a formula to represent the relation between m and n. express m as the subjectof the formula.(ii) 19 matchsticks are used to make the p'th shape. find the value of p.