The number of diagonals in a certain regular polygon is equal to $2$ times the number of sides. How many sides does this polygon have?
Number of diaginals in a regular polygon = (n) (n-3) / 2 .... where n = the number of sides
So
(n) ( n -3) / 2 = 2n
n^2 - 3n = 4n
n^2 - 7n = 0
n ( n - 7) = 0
n - 7 = 0
n = 7
+603 
