d=n^2 - 3n / 2 where d = number of diagonals and n = the number of sides of the polygon. A polygon has 119 diagonals, how many sides does it have? 119 = n^2 - 3n / 2 I am not sure how to do this? can someone please help? sorry this isn't physics related I know I need to use the quadratic formula, but how?