7a²      2a²      b²
————— -  ————— + —————
 12       3b      6a²

The LCD of 12, 3b, and 6a² is 12a²b

Now we ask these questions about the three
denominators:

1.  What does denominator 12 need to be multiplied 
by to get the LCD of 12a²b?

2.  What does denominator 3b need to be multiplied 
by to get the LCD of 12a²b?

3.  What does denominator 6a² need to be multiplied 
by to get the LCD of 12a²b?

Answer to question 1: a²b
Answer to question 2: 4a²
Answer to question 3: 2b

So 

1. We multiply the first fraction by the fraction ,
which equals 1.

2. We multiply the second fraction by the fraction ,
which equals 1.

3. We multiply the third fraction by the fraction ,
which equals 1.
 
 7a²   a²b     2a²   4a²      b²   2b
—————•————— - —————•————— + —————•————
 12    a²b     3b    4a²     6a²   2b

Multiply numerators and denominators of the three 
pairs of fractions to multiply:

 7a4b       8a4        2b³
——————— - ——————— + ———————
 12a²b     12a²b     12a²b

Now that all three fractions have the same
denominator, we can just combine the
numerators over the common denominator:

 7a4b - 8a4 + 2b³
——————————————————
       12a²b

Edwin