let
x--------> the length of a soccer field
y--------> the width of a soccer field
we know that
x=3y-70--------> equation 1
perimeter=2*[x+y]------> 380=2x+2y--------> equation 2
substitute 1 in 2
380=2*[3y-70]+2y-----> 380=6y-140+2y----> 8y=520----> y=65 yd
x=3y-70----> x=3*65-70-----> x=125 yd
the answer is
the length of a soccer field is 125 yd
the width of a soccer field is 65 yd