The thermal energy is the work done by friction. If Justin is at rest intially, his potential energy is MGH. Hence we have 30* 9.8*8 = 2352N. Where m is mass, g accleration due to gravity, and h is the height. When he gets to the bottom of the slide, his potential energy is converted to usable kinetic energy, so we have 1/2 mv^2. We end up with 1/2 * 30 * (11)^2 = 1815N. He started with 2352N and ended up with 1815N. Hence the thermal energy is 2352 - 1815 = 537N.
