[tex]\displaystyle\bf\\a)\\~~9~~1~~7~~3 -\\\boxed{\bf4}~5~\boxed{\bf2}~6=\\~~4~\boxed{\bf6}~4~\boxed{\bf7}\\\\\\b)\\~~2~\boxed{\bf6}~3~\boxed{\bf6}+\\\boxed{\bf6}~9~\boxed{\bf8}~1=\\~~9~\boxed{\bf6}~1~~7\\\\\\c)\\~~8~~0~~0~~0-\\~~1~\boxed{\bf0}~9~~9=\\\boxed{\bf7}~0~\boxed{\bf0}\boxed{\bf1}[/tex]
