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The sum of interior angles of a n-gon is 180*(n-2) degrees.
In your case, equation for unknown number of sides (of vertices) is
180*(n-2) = 540 degrees.
Divide both sides by 180
n-2 = 540/180 = 3
n = 3 + 2.
ANSWER. n = 5: your polygon is a pentagon.
Solved.