Let
x------------> number of chickens
y------------> number of pigs
we know that
chicken------> has 2 feet
pig------------> has 4 feet
then
2x+4y=98--------------------> equation 1
and
x+y=37------------> y=37-x equation 2
I substitute 2 in 1
2x+4[37-x]=98--------------> 2x+148-4x=98
-2x=98-148-------------> 2x=50-------------> x=25
x+y=37------------> y=37-25-------------> y=12
the answer is
25 number of chickens
12 number of pigs