You may have tried to simplify $\displaystyle{f{{\left({x}\right)}}}\usepackage{cmbright}\Huge\$ by first incorrectly subtracting $\displaystyle{8}-{4}{x}\usepackage{cmbright}\Huge\$ to get $\displaystyle{4}{x}+{{x}}^{{3}}\usepackage{cmbright}\Huge\$ . Note that $\displaystyle{f{{\left({x}\right)}}}\usepackage{cmbright}\Huge\$ has unlike terms and cannot be simplified. Start with:

$\displaystyle{f{{\left({x}\right)}}}+{g{{\left({x}\right)}}}={\left({8}-{4}{x}+{{x}}^{{3}}\right)}+{\left({2}{{x}}^{{2}}+{5}{x}-{1}\right)}\usepackage{cmbright}\Huge\$

Rearrange the terms and then combine like terms.