Apet store has 90pets ( dogs, cats, birds) for sale. there are 3 more dogs than cats. there are 6 fewer birds than cats. how many of each type of pets are in the shop? \ i need

There are three unknowns to be found.  We need to find one that can set up the equations easily.

There are 3 more dogs than cats.     
There are 6 fewer birds than cats.

We see that cats (C) would be a useful variable because everything else is related to it.

C+3= number of dogs
C    = number of cats
C-6 = number of birds
Add them all up, we have

3C+3-6=90
Add 3 to each side,
3C=93
C=31    There are 31 cats
C+3=34There are 34 dogs
C-6=25 There are 25 birdsTotal 90 pets.