How to simply 5/3+i

I am assuming you mean 5 / (3 + i)

Multiply top and bottom of the fraction by the complement of 3+i (which is 3-i)

= 5 (3 - i) / (3 + i) (3 - i)

= 15 - 5i / (9 - 3i + 3i - i^2) [Note - i^2 = + 1)

= (15 - 5i) / (9 + 1)

= (15 - 5i) 10

= 1.5 - 0.5i answer