Question

Which matrix can be used to solve the system?

Answer 1

It's the last choice.  In the top row you have the 1's that are the coefficients on the x and the y, and the solution for that equation is 21; in the second row you have the 5 from the x and the 4 from the y and the solution for that equation which is 20.  I'm more used to writing them down instead of across, but there you have it anyways