Question 835561
Let the first side be x, the third side be y, and the longest side be z 
so you have the first side is half the longest side ==> {{{x = z/2}}}
then you have the third side is 12 feet less than the longest side => {{{y = z -12}}} 
Then you have x + y + z = 53 because 53 is the perimeter
that means {{{ z/2 + z - 12 + z = 53}}}
{{{z/2 + 2z = 65}}} 
{{{z + 4z = 130}}}
{{{5z = 130}}}
{{{z = 26}}} 
So now you have z = 26 
{{{x = z/2 = 26/2 = 13}}} 
{{{y = z - 12 = 26 - 12 = 14}}}
SO there are your three sides
TA-DAH
 :D