your mistake is in the numerator of the 3rd expression ...
I used Google Translate for the title because my English isn't that good...
What am I doing wrong?
Here's my attempt to solve this exercise:
And here's the solution:
Can anyone explain me how to solve that right?
The first couple of steps are correct. You have
and correctly recognize that the "3" in that last denominator can be factored out:
so the common denominator is 3b(a-b).
Multiplying both numerator and denominator of: the first fraction by (a-b); the second fraction by 3b; the third fraction by b gives
But now you have as the first two terms in the next fraction and (a+b)(a-b)= . That was probably a typo but you continued it through your answer. You should have
Blast! Skeeter beat me to it again!
Now the " " terms cancel and the fraction simplifies.