1. The problem asks to find the angle number that forms a linear pair with \(\angle SXT\). 2. A linear pair of angles are two adjacent angles whose non-common sides form a straight line (180 degrees). 3. Given the points and lines, \(\angle SXT\) is the angle formed at point X between points S and T. 4. From the description, \(\angle SXT\) corresponds to \(\angle 1\) (between lines T-X and X-S). 5. The linear pair to \(\angle 1\) must share a side and together form a straight line. 6. The line R-X-S is horizontal, so the angle adjacent to \(\angle 1\) on the line X-S is \(\angle 2\) (between X-S and X-U). 7. Therefore, \(\angle 2\) and \(\angle 1\) form a linear pair because their non-common sides (T-X and X-U) form a straight line through X. Final answer: \(\boxed{2}\)