I wasn't in class yesterday for this so- can someone lend a hand? :')

How can you decide witch operations to use to slice a two step equation

Which equation results from adding the equations in this system? x+6y=9 and -x+2y=-15

Louis wants to carpet the rectangular floor of his basement.the basement has an area of 864 square feet.the width of the basement is 2/3 it's length. what is the length of louis's basement