Remove the parentheses and change the signs of the terms of the second polynomial:

$\displaystyle{9}{{a}}^{{2}}-{7}{b}+{3}{{c}}^{{3}}-{4}-{6}{{a}}^{{2}}-{6}{b}+{3}{{c}}^{{3}}+{4}\usepackage{cmbright}\Huge\$

Regroup and combine like terms:

$\displaystyle{9}{{a}}^{{2}}-{6}{{a}}^{{2}}-{7}{b}-{6}{b}+{3}{{c}}^{{3}}+{3}{{c}}^{{3}}-{4}+{4}={3}{{a}}^{{2}}-{13}{b}+{6}{{c}}^{{3}}\usepackage{cmbright}\Huge\$