The domain of function f(x) is [-5,8]. So no matter what the input to "f" is, the domain still applies. In this case the input is "x+2", set input equal to domain to solve for x. These x values will represent new domain for f(x+2) -1.
x+2 = -5 -----> x = -5 -2 = -7 x+2 = 8 ------> x = 8 -2 = 6