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 diagonals in a regular polygon with n sides = (n)(n -3) /2
So
[(n) ( n -3) / 2] = 2n
(n) ( n-3) = 4n
n^2 - 3n = 4n
n^2 - 7n =0
n ( n -7) = 0
n = 0 reject
n = 7
+452 
