Solve 2x + 3y = 21 and x - 2y = -7 by substitution. Check your answer.

Step 1. One variable alone. $x = 2y - 7$.

Step 2. Substitute. $2(2y - 7) + 3y = 21$, so $7y - 14 = 21$ and $y = 5$.

Step 3. Find $x$. $x = 2 \times 5 - 7 = 3$.

Step 4. Check. $6 + 15 = 21$ and $3 - 10 = -7$.