The area of the quadrilateral.
quadrilateral means having all sides the same length. when you make a shape with 3 obtuse angles and with the same length on the sides, only 3 of them will have the same length which is the sides of the 3 obtuse angles. then the last side will be too long. obviously it is no more quadrilateral.
Area
You could be like the side elevation of a staircase.
Which quadrilateral? What is x?
the length of one side times by four
An isosceles trapezoid
A regular quadrilateral has its 4 sides of the same length. If the perimeter (sum of the 4 sides) is 36 it means one side is a fourth of that, or 9.
rhombus
A "regular quadrilateral" is a square. The word "radius" isn't used for squares (or polygons in general). To get the length of a side, divide the perimeter by 4. To get the diagonal of a square, multiply the length of a side by the square root of 2.
square
A regular pentagon with side length 26 cm has a perimeter of 26*5 = 130 cm. The three sides of the quadrilateral sum to 98.6 cm so the fourth side must be 130-98.6 = 31.4 cm.