Question 828228
Triangle:  x = length of a side, perimeter is 3x.


Rectangle:  side length is x+4; side width is x-6.
perimeter is 3x; {{{2(x+4)+2(x-6)=3x}}}, because given both perimeters are equal.


{{{2x+8+2x-12=3x}}}
{{{x+8-12=0}}}
{{{x=12-8}}}
{{{x=4}}}----final answer, but not a formula.  A side length for the equilateral triangle.


You could start with the perimeter equation for the rectangle and adjust as you want if you want a formula.