To find the angle $x$ in the given right triangle, we can use the trigonometric function tangent, defined as:

$\tan(\theta) = \frac{\text{opposite}}{\text{adjacent}}$

Here, the side opposite to angle $x$ is 15, and the side adjacent to $x$ is 21. Therefore, we have:

$\tan(x) = \frac{15}{21}$

To find $x$, we take the arctangent (inverse tangent) of $\frac{15}{21}$:

$x = \arctan\left(\frac{15}{21}\right)$

Let's calculate this:

$x \approx \arctan(0.7143) \approx 35.5^\circ$

So, $x \approx 35.5^\circ$ when rounded to the nearest tenth of a degree.

Would you like a more detailed explanation or have any further questions?

Here are some related questions for further exploration:

1. What is the sine of angle $x$ in this triangle?
2. How would you find the hypotenuse using the Pythagorean theorem?
3. Can you determine the other acute angle in the triangle?
4. How would the calculation change if the sides were labeled differently?
5. What other trigonometric functions can be used to find angle $x$?

Tip: Always double-check which sides are opposite and adjacent relative to the angle you are finding.