We recall that the angle formed by a tangent to a circle and the radius drawn to the tangency point is 90°.
So, the measures of the interior angles of the quadrilateral are x°-90°-90°-135°.
Thus, the total sum of these angles is x°+90°+90°+135°=315°+x°.
Since the sum of the measures of the angles of a quadrilateral is 360°, then x must be 45.
Answer: 45