The answer is -5/12.
You have to look for the LCD of 3 and 4. 
Get the equivalent of the first fraction to 12, which is 4/12.
Then get the equivalent of the second fraction to 12, which is -9/12.
Proceed to the addition rules of similar fractions.

Always remember to have your fractions in the same denominator in order to perform the operations easily. Pactice too in in getting the LCD faster.