I don't know if the asker meant those to all be in Fahrenheit; let's assume it's correct as written.
The interesting thing that this will teach us is that around -40 degrees the Celsius and Fahrenheit scales are about the same.
F=-31
C = (5/9) (F - 32) = (5/9)(-31 - 32) = (5/9)(-63) = 5(-7) = -35
Answer for a: -31°F = -35°C
The next two give us C and we want F. Let's solve the general formula for F.
C = (5/9) (F - 32)
(9/5) C = F - 32
F = (9/5) C + 32
C = -45
F = (9/5)(-45) + 32 = -81 + 32 = -49
Answer for b: -45°C = -49°F
C = -40
F = (9/5) C + 32 = (9/5) (-40) + 32 = -72 + 32 = -40
They're the same!
Answer for c: -40°C = -40°F
