Choose the regular polygon that has interior angles with a sum of 900 degrees.

A triangle has 3 sides, the total interior angels sum to 180 degrees.
A square has 4 sides, the total interior angels sum to 360 degrees.
A pentagon has 5 sides, the total interior angels sum to 540 degrees.
Do you see a trend?

The (s) sum of interior angles equals the (n) number of sides, minus 2, times 180.
This is written as
s = (n - 2) * 180

If we put in 900 for s, we get
900 = (n - 2) * 180

5  = n - 2
n = 7
A polygon with 7 sides is called a Heptagon.