Which rule explains why these triangles are congruent?

Solve 2x+y=3 x+y=5 in substitution method

(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.

The area of each triangle is 7 square units. the surface area of the triangular prism is