Answer
$ 24.6\times10^{5} m$
Work Step by Step
When we are adding two physical quantities both of them should have the same units. Therefore we can convert kilometers into meters as follows.
$2.1\times10^{3} km = 2.1\times10^{3}\times10^{3} m = 2.1\times10^{6} m$
Then,
$3.6\times10^{5} m + 2.1\times10^{3} km = 3.6\times10^{5} m + 2.1\times10^{6} m = (3.6\times10^{5} + 21\times10^{5}) m$
$=(3.6+21)\times10^{5} m$
$=24.6\times10^{5} m$
