You may have tried to simplify $\displaystyle{f{{\left({x}\right)}}}\usepackage{cmbright}\Huge\$ by first subtracting $\displaystyle{6}-{3}\usepackage{cmbright}\Huge\$ to get $\displaystyle{3}{{x}}^{{2}}\usepackage{cmbright}\Huge\$ . However, you have used an incorrect order of operations. 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({6}-{3}{{x}}^{{2}}\right)}+{\left({4}{{x}}^{{2}}-{x}-{2}\right)}\usepackage{cmbright}\Huge\$

Rearrange the terms and then combine like terms.