Question

Which matrix can be used to solve the system?
x+y=21
5x+4y=20

A)
[x]=[4 -1][21]
[y]=[-5 1][20]

B)
[x]=[-4 1][21]
[y]=[5 -1][20]

C)
[x]=[-1 1][21]
[y]=[5 -4][20]

D)
[x]=[1 1][21]
[y]=[5 4][20]

If there is any confusion on what anything written here is here is a screenshot of everything.
https://postimg.cc/image/rhrj0jwnr/

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