How to Add Fractions with different denominators:

Find the Least Common Denominator (LCD) of the fractions
Rename the fractions to have the LCD
Add the numerators of the fractions
Simplify the Fraction

Example: Find the Sum of 2/9 and 3/12

Determine the Greatest Common Factor of 9 and 12 which is 3
Either multiply the denominators and divide by the GCF (9*12=108, 108/3=36)
OR - Divide one of the denominators by the GCF and multiply the answer by the other denominator (9/3=3, 3*12=36)
Rename the fractions to use the Least Common Denominator(2/9=8/36, 3/12=9/36)
The result is 8/36 + 9/36
Add the numerators and put the sum over the LCD = 17/36
Simplify the fraction if possible. In this case it is not possible