First, we need to find the last interior angle of the triangle. Since we know the exterior angle next to it is 140° and they are supplementary, meaning they form a 180° angle, we can make an equation to solve for the last angle.
∠SRT + 140° = 180°
∠SRT = 40°
Now we know that the last interior angle is 40°. All triangles have a sum of 180°, therefore we can once again set up an equation to solve for x.
