Those two quantities cannot be added.
In order to add or subtract quantities, they don't necessarily both need to
have the same units, but both must have the same physical dimensions
so that one unit can be converted to be the same as the other unit.
"Physical dimensions" are things like mass, length, area, volume, time,
force, speed, etc.
In this problem, one quantity has the unit 'kg'so its physical dimension is mass.
The other quantity has the unit 'km' so its physical dimension is length. Obviously
these physical dimensions are different, so there's no way that either unit can be
converted to match the other one, and the quantities can't be added.
