since 3 poles is $120 and 4 poles is $160
you can find the difference between the two to find the price of one pole
160-140 = 20
each pole is $20
you can then do the equation
$120 = 20(3)+ y
3 being the amount of poles and y being the fee added to it.
120 = 60 + y
y = 60
so the final answer would be c = 20n + 60