Afarmer has chickens and cows. you ask the farmer how many chickens he has, and how many cows he has. the farmer has 28 healthy animals, and they have total of 64 legs. how many of his animals are cows? write an equation

Set the number of chickens as x and number of cows as y
so the farmer has total 28 animals
so x+y=28 
you know that since the animals are healthy
chickens will have 2 legs    and
cows will have 4 legs
the total number of legs are 64
so 2x+4y=64
since you want to find the number of cows 
so from the first equation x+y=28
we will get another equation x=28-y
then plug into 2x+4y=64
2(28-y)+4y=64
56-2y+4y=64
56+2y=64
-56      -56
2y=8
y=4

What can we do about it