recall that a circle has a total of 360°, now the arc that is NOT part of "x", is 250°, therefore the leftover, namely the arc made where "x" is at, has to be 360° - 250°, that's the degree measure of that arc.
now, arc's get their angle measure from the central angle they're in, in this case, "x" is the central angle, and therefore, it has the same measure of 360° - 250°.