Hi,
The sum of the measures of the interior angles of a polygon with x sides is equal to (x-2) times 180 degrees. A pentagon has got 5 sides, so the sum of the measures of its interior angles is 180(5-2) = 180(3) = 540 degrees.
I hope this helps. If I was not clear enough or if you’d like further explanation please let me know. Also, English is not my first language, so I’m sorry for any mistakes.