Solve the Problem $$\frac { 1 } { 2 } n ( n - 3 )$$

Multiple Choice

Solve the problem.
-If a polygon, of n sides has $$\frac { 1 } { 2 } n ( n - 3 )$$ ) diagonals, how many sides will a polygon with 189 diagonals have?

A) 22 sides
B) 21 sides
C) 20 sides
D) 23 sides