Question:
Solve 19 = -13*f - 20 for f..

Answer:
-3

Question:
Solve 336*y + 2391 = -297 for y..

Answer:
-8

Question:
Solve 108*f = 93 - 309 for f..

Answer:
-2

Question:
Solve -90*o + 509 + 1069 = 138 for o..

Answer:
16