Easiest way is to make a cut out of the image and determine if it folds in half. The crease made by folding in half is the line of symmetry.
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2 lines of symmetry
A nephroid has 2 lines of symmetry.
A parallelagram can be a square, which has four lines of symmetry or a rectangle which has two lines of symmetry but the generic parallelagram has zero lines of symmetry
5 lines
A decagon need not have any lines of symmetry. It can also have 1 or 10 lines of symmetry.