documentclass[12pt,letterpaper]{article}

usepackage{exsheets}

% answer space macro
newlength{ALT} %Sets name new my length, where ALT = Answer Line Thickness
setlength{ALT}{0.5pt} %sets thickness of that line

newlength{DLL} % DLL = default line length; that is if I don't otherwise specify the line length, it will be as long as I set it to here.
setlength{DLL}{1cm}

newcommand{AS}[1][DLL]{rule{#1}{ALT}} %AS = Answer Space

begin{document}

begin{enumerate}
item (3=10-) AS
item (2+6=) AS
item (15-) AS (=7)
item (2+3=) AS
end{enumerate}

end{document}