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

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