It depends on what information you have.
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Measure it!
It depends on the information available to you.
The answer will depend on whether the length is the hypotenuse or one of the legs of the triangle.
You first multiply the length and the width, the you divide it by 2.
To find the length of a triangle or what ever shape your trying to find all you have to do is multiply width times the area. * * * * * Or, if you want the correct answer, you could try to divide the area by the width. That assumes the shape is a rectangle and that the area and width are known!