cost function C = 25x + 7000, revenue fcn R = 40x

[tinad]

the cost and revenue function for a product are C=25x+7000 and R=40x. what is the limit of the average profit function p=R-C/x as x-->inf.

would I start like this p=40x-25x+7000/x then p+15x+7000/x then p=15x-7000x^-1 then P' = 15-7000x^-2
x^2=7000=0
x=sq root 7000 =83.66
Help

 

[skeeter]

P_avg = [R(x) - C(x)]/x

P_avg = [40x - (25x + 7000)]/x

P_avg = (15x - 7000)/x

P_avg = 15 - 7000/x

lim{x -> inf} 15 - 7000/x = ?