First of all, you need to remember that all triangles are equal to 180°. By examining the triangle, you already know two of the angles: 45° and 90° (you can tell it is a right triangle because of the box in the bottom-right corner). Since you have two of the three angles, you can deduce that the unknown angle is 45° (45 + 90 + <u>45</u> = 180).
(Just in case you need to also find x, just write it as an equation and solve for x.)