Q: Solve:
Solve 0 = 20*a - 21*a - 4 for a.

A: -4

Q: Solve:
Solve 0 = -71*r + 31279955 - 31284925 for r.

A: -70

Q: Solve:
Solve 14823 = 67*r + 15627 for r.

A:
-12