Can someone show me how to do this?

For a expression to be completely simplied all roots have to be in the numinator (including i) so you multiply by the conjugate, in this case 5-i, to pul it out of the denominator. So 2(5-i)/(5+i)(5-i) = (10-2i)/25-i^2 = 2(5-i)/26 = (5-i)/13 so C