1. **Problem statement:** We need to find the length of the unknown side $x$ in a right triangle where the other two sides are 2 and 5. 2. **Formula used:** According to the Pythagorean theorem, in a right triangle, the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. The formula is: $$x^2 = a^2 + b^2$$ where $x$ is the hypotenuse, and $a$ and $b$ are the other two sides. 3. **Apply the formula:** Here, the sides are 2 and 5, so: $$x^2 = 2^2 + 5^2$$ 4. **Calculate the squares:** $$x^2 = 4 + 25$$ 5. **Sum the squares:** $$x^2 = 29$$ 6. **Solve for $x$ by taking the square root of both sides:** $$x = \sqrt{29}$$ 7. **Interpretation:** The length of the unknown side $x$ is $\sqrt{29}$, which is approximately 5.385. None of the options A ($\sqrt{10}$), B (10), C (7), or D ($\sqrt{7}$) match this value exactly. **Final answer:** $x = \sqrt{29}$