Help please if anybody knows this help!!

The pyramid below was dissected by a horizontal plane which shape describes the pyramid horizontal cross section

I'm confused on this could anyone me and show me what i need to do to solve it.

(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.