Add I to both sides of the equation: I+58=27, now subtract 58 from both sides: I=-31, so that's the answer, I=-31. If you have problems understanding negatives, picture a thermometer with temperature values above freezing (0 degrees) and temperatures below zero. Above zero the temperatures are positive and below zero they're negative. Subtracting 58 degrees from 27, you subtract 27 first to get to zero, then a further 31 takes you to 31 degrees below zero.