1. The question asks why we choose $x$ instead of $y$ or $z$ for step 5. 2. In algebra or problem-solving, the choice of variable often depends on clarity, simplicity, or the problem's context. 3. Choosing $x$ might be because it is the standard variable for unknowns, making the solution easier to follow. 4. Sometimes $y$ or $z$ are reserved for other variables or dimensions, so using $x$ avoids confusion. 5. The key is consistency and clarity in the steps, so selecting $x$ helps maintain that. 6. Therefore, $x$ is chosen for step 5 to keep the explanation straightforward and avoid mixing variables.