...
2x+4y+6a+0b = 20
2x+8y+0a+6b=40
We need to find the values for a and b. Problem is we have three equations and four unknowns.
Write an augmented matrix using the above equations.
...
\left|\begin{array}{cccc | c} 0 & 0 & 7 & 0 & 15 20 // 1 & 2 & 4 & 6 3 & 0 & 20 10 // 2 & 8 & 0 & 6 & 40 \end{array}\right|