Ok, here's the goal. You have to multiply every fraction to be added (just like with the numerical case) by something equal to 1, such that the new denominator is equal to the "least common multiple". You would agree, I hope, that anything of the form x/x is equal to 1, provided x is not zero, right? So, if I'm adding up I determine that the least common multiple is 6, as you've pointed out. Then I multiply each fraction by what's missing (and in order not to change what the fraction is, I multiply by something that looks like x/x) from the LCM. So try to follow this:

The expressions in parentheses are all equal to 1, and since I'm multiplying each fraction by an expression equal to 1, you would agree I haven't changed the fraction numerically, right? This is exactly the same sort of thing I need to do with the addition problem in post # 28. Check this out:

Again, the expressions in parentheses are all equal to 1 (provided the denominators are not equal to zero). We continue:

Did you follow that? And can you perform the next step in the addition problem?