documentclass[12pt]{article}
usepackage{amsmath}

begin{document}

$$frac{1}{8}cdotunderline{hspace{1.5cm}}=underline{hspace{1.5cm}}
hspace{1cm}
frac{7}{12}cdotunderline{hspace{1.5cm}}=underline{hspace{1.5cm}}$$

end{document}