You may have reversed the order of the functions when subtracting.

$\displaystyle{\left({h}-{g}\right)}{\left({x}\right)}={h}{\left({x}\right)}-{g{{\left({x}\right)}}}=-{4}{{x}}^{{2}}+{6}{x}+{2}-{\left({10}{{x}}^{{3}}-{8}{x}-{9}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle=-{4}{{x}}^{{2}}+{6}{x}+{2}-{10}{{x}}^{{3}}+{8}{x}+{9}=-{10}{{x}}^{{3}}-{4}{{x}}^{{2}}+{6}{x}+{8}{x}+{2}+{9}\usepackage{cmbright}\Huge\$

$\displaystyle=-{10}{{x}}^{{3}}-{4}{{x}}^{{2}}+{14}{x}+{11}\usepackage{cmbright}\Huge\$