Question 174090
You are given 4 sides on the rectangle. Three on the triangle.
The perimeter of a polygon is the sum of its sides.
{{{PerimeterRectangle = 2x + 2x + 5 + 5}}} = {{{4x + 10}}}
{{{PerimeterTriangle = 3x + 3x + 6}}} = {{{6x + 6}}}

You are told the two polygons have the same perimeter. So
{{{4x + 10 = 6x + 6}}}  collect like terms
{{{4 = 2x}}}
{{{2 = x}}}

So the rectangle is 4 x 5
and the triangle is equilateral (all 3 sides are a length of 6.
Each has a perimeter of 18