Question 1065412
{{{D=2+5M}}}
and
{{{D+M=92}}}
Substituting,
{{{(2+5M)+M=92}}}
{{{6M+2=92}}}
{{{6M=90}}}
{{{M=15}}} 
Now that you know Mark has 15 marbles, use either equation to solve for D, the number of Don's marbles.