Find the lcd that would eliminate the fractions in this equation
3a/7 + 2/3 = 7/9

Least Common Denominator is best found by listing out the multiples of the biggest denominator you have, in this case, 9. As you do it,, check it with the others by trying to divide, if it works, then that is a multiple as well:

9: 9,18,27,36,45,54,63,72, etc...

Now in my head, I'd be looking at it like this: 
"7 is my next biggest number, so does 9 divided by 7 work? no. 18 divided by 7? no" and so on until I find on that works, in this case, it's 63. Now we check it with the next lowest number, 3. 63 divided by 3 works, so the LCD is 63