So,
To find the number of sides of the polygon, we could use the fact that:
The formula for calculating the sum of interior angles is ( n − 2 ) × 180 where n is the number of sides. All the interior angles in a regular polygon are equal.
Given that the sum of all the internal angles is 1620, We could write the following equation and then solve for n as follows:

Therefore, the number of sides is 11.