Answer:
the sum of all interior angles in a polygon, regular or not, is
18(n - 2)
where "n" is how many sides are there in the polygon, now, notice, this polygon has 7 sides, so is a heptagon, so the sum of its interior angles will be 18(7 - 2) or 900.
now, subtract all other angles from 900, and what's leftover is "x".