Question: What is the solution?

Solve 0 = 42*d - 51*d + 9 for d.

Answer: 1

Question: What is the solution?

Solve 421*i + 934*i = 58*i - 19455 for i.

Answer: -15

Question: What is the solution?

Solve -1554 = 380*o + 39*o - 9096 for o.

Answer: 18

Question: What is the solution?

Solve 9*i + 23*i + 412 = -36 for i.

Answer:
-14