Look up "sum of interior angles of a polygon" on Google. There you will find a formula for predicting the number of interior angles that a given polygone will have. This formula is (n - 2) × 180°, where n represents the number of sides of the polygon in question. Count these sides. Substitute your value for n into this formula. The interior angles of your polygon add up to the result of evaluating this formula. Call this "sum." Then:
"sum" = (the one unknown interior angle) + (the sum of the given (known)) interior angles).