You first need to find the lowest common denominator. In this case, the lowest common denominator is 30, the lowest number that both 5 and 6 evenly divide into. The next step is the translate the fractions into 30ths. 6 goes into 30 five times, so we need to multiply the numerator, 9, by 5. For the second fraction, 5 goes into 30 six times, so the numerator, 3, should be multiplied by 6.
What does that result in?
-45/30+(-18/30) =-63/30 . Simplifying that, we end up with -2 1/10
