Math Help - fundamental pythagorean triangles

1. fundamental pythagorean triangles

find all fundamental pythagorean triangles whose areas are three times their perimters.

2. Use the fact that if $a^2+b^2=c^2$ then $a=m^2-n^2$, $b=2mn$, $c=m^2+n^2$ for some integers $m,n$.

3. Ive been trying that but I guess ive been doing it wrong.
if a=mn b=m^2-n^2 and c=m^2+n^2
and i want the area to be three times the perimeter.

Area=ab/2 = (mn)*(m^2-n^2) *1/2
Perimeter = mn + m^2-n^2 + m^2 + n^2 = mn + 2m^2

I dont really understand what i would do now...??