Question 964310
The equation for the perimeter is {{{2a+2b=P}}}, so plug in what you know into the equation.
{{{2(7a)+2(10a+12)=31}}}
Now solve for a.
{{{14a+20a+24=31}}}
{{{34a+24=31}}}
{{{34a=7}}}
{{{a=(7/34)}}}