Yaster Car Sharing offers a membership plan with a $45 per month fee that includes 9 hours of driving each month and charges $9 for each additional hour. Let F(x) be the cost for a month in which a member uses a car for x hours. Then F(x) is the piecewise function F(x)=45, for 0 < x < 9. F(x) =mx+b, for >9 for some scalars m and b. Find b. Round to the nearest whole number.