While subtracting g from f, you may have combined the two middle unlike terms of the functions. Set up the problem as follows:

$\displaystyle{\left(\frac{{{f{-}}{g}}}{{h}}\right)}{\left({x}\right)}=\frac{{{f{{\left({x}\right)}}}-{g{{\left({x}\right)}}}}}{{{h}{\left({x}\right)}}}=\frac{{{10}{{x}}^{{3}}-{15}{{x}}^{{2}}-{5}-{\left(-{10}{{x}}^{{3}}-{10}{x}-{5}\right)}}}{{{5}{x}}}\usepackage{cmbright}\Huge\$

Be careful to distribute the minus sign to all terms in g. Combine like terms only, factor the numerator, and simplify.