Solve this system of equations for $x$ and $y$ in terms of $a$ and $b$. \begin{align*} y - 3x &= a, \\ x &= 2y + b. \end{align*}
+37146 y = a + 3x
x = 2y + b sub in the first equation for y
= 2(a+3x) + b
x = 2a + 6x + b
-5x = 2a + b
x = -1/5 ( 2a + b) sub this into the top equation y = a - 3/5 (2a+b) and simplify a bit ....
