Common denominators are two fractions put on a same denominator... (since I'm bad at explaining, I'll give an example) If I wish to add 2/3 to 2/12... I'd go : 2/3 x 4 which gives 8/12 + 2/12 = 10/12...
One easy way is to just multiply all denomonators togother and make THAT yurr comon denomonator but the BEST way is to find all the FACTORS of yurr denomonators :)