帮我讲解图片中的数学题---**Extraction Content:** **Question Stem:** 关于x的不等式3x - a ≥ x + 1的解集在数轴上表示如图所示, 则a的值是_. **Mathematical Formula/Inequality:** 3x - a ≥ x + 1 **Chart Description:** * **Type:** Number Line graph. * **Main Elements:** * A horizontal number line with an arrow pointing to the right (positive direction). * Labeled tick marks at integer values from -2 to 3. * A solid black circle (filled dot) located at the position labeled '1' on the number line. * A horizontal line segment starting from the solid circle at '1' and extending to the right, above the number line. The end of this line is not explicitly marked but extends in the positive direction. * Labels: -2, -1, 0, 1, 2, 3 below the corresponding tick marks. * **Interpretation:** The graph represents the solution set of an inequality, indicating that all numbers greater than or equal to 1 are included in the solution. The solid dot at 1 means that 1 is included. The line extending to the right indicates all values greater than 1 are included. This corresponds to the inequality x ≥ 1.