It's +31.
if there's a minus next to a bracket you have to turn every operator the opposite inside that bracket
-(-31) = +31
That equation can represent the relationship among the image distance,
object distance, and focal length of a lens.
It could describe the effective resistance of two resistors in parallel, the
effective inductance of two coils in parallel, or the effective capacitance
of two capacitors in series.
Thanks for sharing it. Now, did you have a question to ask ?
