I need help answering this question for my geometry homework

What is the missing reason in step 5? linear pair postulategivendefinition of complementary anglescongruent complements theorem

Mary works for a company that ships packages and must measure the size of each box that needs to be shipped. mary measures a box and finds the length is 7 inches, the width is 14 inches, and the height is 15 inches. what is the volume of the box? [type your answer as a number.]

Simplify a7/a4 a) 1/a3 b)-a/a2 c)a4/a3 d)a3