Help Me please.

This is very similar to all of your other postings over the last few days...are you catching on yet??????

This one is (assuming there is a parentheses around the numerator) ....

a^-2 / a^2b^-1     +   a^-1b / a^2b^-1    -   b^2 / a^2b^-1

b / a^4               +      b^2 / a^3             -   b^3 /a^2  

a^-2 b  (  a^-2    +      b a^-1                  -   b^2)

Never mind

Manuel says that this is actually 

a^-2 + a^-1b - b²/ (a²b^-1 )

So we have

a^-2 + a^-1b   - b²* b * a^-2

a^-2 + a^-1b - b³*a^-2       factor out   a^-2

a^-2  [ 1  + ab - b³ ]