Distribute the subtraction sign to both terms in the subtrahend. Rearrange the terms to put like terms together and then combine like terms.

$\displaystyle{\left({6}+{8}{i}\right)}-{\left({3}+{5}{i}\right)}={6}+{8}{i}-{3}-{5}{i}={6}-{3}+{8}{i}-{5}{i}={3}+{3}{i}\usepackage{cmbright}\Huge\$