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

$\displaystyle{4}{{x}}^{{4}}+{2}{{y}}^{{3}}-{6}{{z}}^{{3}}+{4}+{9}{{x}}^{{4}}+{3}{{y}}^{{3}}-{4}{{z}}^{{3}}-{9}\usepackage{cmbright}\Huge\$

Regroup and combine like terms:

$\displaystyle{4}{{x}}^{{4}}+{9}{{x}}^{{4}}+{2}{{y}}^{{3}}+{3}{{y}}^{{3}}-{6}{{z}}^{{3}}-{4}{{z}}^{{3}}+{4}-{9}={13}{{x}}^{{4}}+{5}{{y}}^{{3}}-{10}{{z}}^{{3}}-{5}\usepackage{cmbright}\Huge\$