It is TRUE that this polygon has 7 sides.
However, it is NOT TRUE - as the other person stated - that each interior angle is, since itw's never indicated that the polygon is REGULAR.
You write the equation as you read the problem 900 = 180*(n-2) degrees (the sum of interior angles of an n-gon) You cancel the factor 180 in both sides 5 = n-2. From it, you find the ANSWER to your problem n = 5 + 2 = 7.