Question 847024
Something is wrong with your problem setup.
{{{S=M-21}}}
In 6 years, she will be M+6 and the son will be S+6.
{{{M+6=5(S+6)}}}
Substitute eq. 1 into eq. 2,
{{{M+6=5(M-21+6)}}}
{{{M+6=5(M-15)}}}
{{{M+6=5M-75}}}
{{{4M=81}}}
{{{M=81/4}}} 
Then the son would be,
{{{S=81/4-21}}}
{{{S=81/4-84/4}}}
{{{S=-3/4}}}
Typically the solutions are integers first off and secondly no negative ages are allowed.
I also changed the problem in case the mother is five times older than his age today.
{{{M+6=5S}}}
Then,
{{{M+6=5(M-21)}}}
{{{M+6=5M-105}}}
{{{4M=111}}}
{{{M=111/4}}}
and then 
{{{S=111/4-84/4}}}
{{{S=27/4}}}
Again typically the solutions are in integers values. 
Please check the problem and repost if needed.