It appears that you added instead of subtracting. Distribute the subtraction sign to both terms in the subtrahend.

$\displaystyle{\left(-{4}+{3}{i}\right)}-{\left(-{6}+{4}{i}\right)}=-{4}+{3}{i}+{6}-{4}{i}\usepackage{cmbright}\Huge\$

Rearrange the terms to put like terms together, and then combine like terms.