In 2009 the U.S. Postal Service charged $1.17 postage for first-class mail retail flats (such as an 8.5'' by 11 envelope) weighing up to 1 ounce, plus $0.17 for each additional ounce up to 13 ounces. First-class rates do not apply to flats weighing more than 13 ounces. Develop a model that relates C, the first-class postage charged, for a flat weighing x ounces. Graph the function.