To be able to add miles and kilometres together you have to convert one of the units to the other.
For result in miles multiply the kilometres by 0.62137119
For results in kilometres multiply the miles by 1.609344

Results in Kilometres:  2.3 km + 3.6 mi
2.3 + (3.6 x 1.609344) = 2.3 + 5.794 =  8.094 kilometres

Results in Miles: 2.3 km + 3.6 mi
(2.3 x  0.62137119) + 3.6 =  1.429 + 3.6 = 5.029 miles