Depends on the problem; many 3D figures have 2D faces/shapes within them. One can say "find the perimeter of a cylinder," and the question could be interpreted as finding the perimeter of one of the circular bases of the cylinder. Use common sense, and find out what the question is asking.
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It isn't clear how you would define the "perimeter" for a 3D figure. The base is the product of length x width.
You find perimeter by adding all the sides of a figure.
If the figure is a polygon ... with sides made of straight line segments ... then the perimeter is the sum of the lengths of all the sides. If part or all of the figure's boundary consists of curves, the perimeter is still the distance all around the figure, but you may need special formulas to find the lengths of the curved sections.
To find the perimeter of a figure, add up the lengths of all of its sides.
If the figure is 2-dimensional, it is called the perimeter.