Solving systems by elimination what's the answer?

Solve the system of equations. y= 2x y= x^2 - 15

Ineed if you could explain and give me the answer you! this needs done

Mr. brettler is hanging up a 5-foot tall rectangular welcome sign in the gym. he has drawn a sketch on a coordinate grid. he has the lower vertices of the sign at (9, –3) and (–6, –3). where would the upper vertices be on the grid?