Draw the lines x + 2y = 9 and 3x - y = 6 on one graph.

Step 1. Points on the lines. $x + 2y = 9$ passes through $(9, 0)$ and $(1, 4)$. $3x - y = 6$ passes through $(2, 0)$ and $(0, -6)$.

Step 2. The crossing. Substituting $y = 3x - 6$ into the first: $x + 6x - 12 = 9$, so $7x = 21$, $x = 3$ and $y = 3$.

Step 3. Check. $3 + 6 = 9$ and $9 - 3 = 6$.