Okay so if you want to learn more about questions like this search for head and feet math problems.
We know that the woman has 21 pets which are either a cat or a bird and the total number of legs is 76.
We know that a cat has 4 legs and a bird has 2 feet
So we assume that all pets are cats
If this is so the total number of legs would be 4*21=84
The difference between the number of feet the bird and cat has is 2
In order to get the number of gets the expression would be:
Therefore there are 4 birds and 21-4=17 cats.