You may have thought that adding two functions means to combine all terms of both functions together. The notation should be interpreted as follows:

$\displaystyle{\left({f{+}}{g}\right)}{\left({x}\right)}={f{{\left({x}\right)}}}+{g{{\left({x}\right)}}}={\left({6}{x}\right)}+{\left({3}{x}+{3}\right)}\usepackage{cmbright}\Huge\$

Now combine like terms.