Lou’s latest invention, aimed at urban dog owners, is the x-r-leash. it is made of a rubber-like material that exerts a force fx = (−7.9 mm) x − (69 n/m2 ) x 2 when it is stretched a distance x. the ad claims, "you’ll never go back to your old dog leash after you’ve had the thrill of an x-r-leash experience. and you’ll see a new look of respect in the eyes of your proud pooch." find the work done on a dog by the leash if the person remains stationary, and the dog bounds off, stretching the x-r-leash from x = 0 m to x = 20 m. answer in units of kj.