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It can be any 2-dimensional figure.
A triangle cannot have two parallel sides. In order for a figure to be a triangle, it must be a closed figure with three sides and three angles. If two of the sides were parallel, there is no way the figure could be closed by only one other line.
It could be a trapezoid providing that the other two angles are acute and obtuse
Then the figure has line symmetry.
If none of the sides ever cross each other, there cannot be a closed plane figure.