1. **State the problem:** You asked why angle $x$ is $90^\circ$ when the triangle is not right-angled. 2. **Recall the property of a triangle inscribed in a circle with diameter as one side:** If a triangle is inscribed in a circle and one side of the triangle is the diameter of the circle, then the angle opposite that side is a right angle ($90^\circ$). 3. **Explanation:** Since $AB$ is the diameter of the circle, the angle at vertex $C$ (which is opposite $AB$) must be $90^\circ$ by Thales' theorem. 4. **Conclusion:** Even if the triangle does not look right-angled at first glance, the inscribed angle subtending the diameter is always a right angle. Therefore, $x = 90^\circ$ is correct.