Simplify.

$\displaystyle{\left({3}{{x}}^{{2}}+{x}+{7}\right)}+{\left({10}{{x}}^{{3}}-{2}{{x}}^{{2}}-{8}{x}-{12}\right)}={10}{{x}}^{{3}}+{{x}}^{{2}}\usepackage{cmbright}\Huge\$ ___ $\displaystyle-{5}\usepackage{cmbright}\Huge\$

Fill in the blank with the missing term. Include the symbol - if the term is negative.