Treat the division as a fraction, rationalize the denominator by multiplying both the numerator and the denominator by the complex conjugate of the denominator, and, if possible, simplify.

$\displaystyle\frac{{{7}-{5}{i}}}{{{1}-{i}}}={\left(\frac{{{7}-{5}{i}}}{{{1}-{i}}}\right)}\cdot{\left(\frac{{{1}+{i}}}{{{1}+{i}}}\right)}=\frac{{{7}+{7}{i}-{5}{i}-{5}{{i}}^{{2}}}}{{{1}+{i}-{i}-{{i}}^{{2}}}}\usepackage{cmbright}\Huge\$

$\displaystyle=\frac{{{12}+{2}{i}}}{{{2}}}={\left(\frac{{{6}+{i}}}{{{1}}}\right)}\cdot{\left(\frac{{2}}{{2}}\right)}={6}+{i}\usepackage{cmbright}\Huge\$