The answer depends on what the shape is: triangle, quadrilateral or one with many more sides. You do need to specify these things in the question because, unfortunately, we have not mastered the art of reading your mind over the web.
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you must know more information. Like the lengths of 2 sides. Then using Trig (law of sines or law of cosines) you can find the remaining sides and angles.
The answer depends on what the problem is: are you required to find the lengths of the missing sides, the area, angles, length of diagonal, or WHAT!
Add together the lengths of the four sides; or add the lengths of the two different sides and then double it.
That really depends what else you know about the triangle. For example, if you only know the lengths of two sides, you simply don't have enough information.
Find the perimeter by adding the lengths of the sides