Use (n - 2) × 180 to find the sum of the measures of the interior angles of the polygon, where n = number of sides of the polygon. The polygon has 7 sides, so plug 7 in for n.
(7 - 2) × 180 = 900 The sum of the interior angles of this 7-gon is 900°.
Now that we know this, make an equation where all the interior angles add up to 900, then solve for x.