We can write, 5/-8 = 5 x (-1)/-8 x (-1) = -5/8 [multiplying numerators and denominators by (-1)]
15/-28 can be written as = 15 x (-1)/-28 x (-1) = -15/28
and -17/-13 can be written as = -17 x (-1)/-13 x (-1) = 17/13, as both negative signs are cancelled.