Short answer: the rectangle is a square, so the length and width are ...
(316 m)/4 = 79 m.
_____
Let x represent the length of the rectangle. Then the width in meters is 316/2 -x. The area is the product of length and width.
area = x(158 -x)
This is the equation of a parabola that opens downward. It has zeros at x=0 and x=158. The vertex is on the line of symmetry, halfway between the zeros, at x=79.
The length is 79 m.
The width is 158 -79 = 79 m.