It appears that you multiplied the two functions, perhaps because of the parentheses used in the function notation. The notation should be interpreted as follows:

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

Now combine like terms.