Find a, b, c, d if a - b, 2a + c; 2a - b, 3c + d = -1, 5; 0, 13.

Step 1. The equations. $a - b = -1$, $2a + c = 5$, $2a - b = 0$ and $3c + d = 13$.

Step 2. Find $a$ and $b$. Subtract the first from the third: $a = 1$. Then $b = a + 1 = 2$.

Step 3. Find $c$ and $d$. $2 + c = 5$ gives $c = 3$. Then $9 + d = 13$ gives $d = 4$.