First, we will find what the value of U is before we slide the number line to the correct position:
[tex]\begin{gathered} U=a-b \\ a=-1 \\ b=-2 \\ U=-1-(-2) \\ U=-1+2 \\ U=+1 \end{gathered}[/tex]So to get the number line position, we will slide the green dot from -1 position, two units to the right, and stop at +1 position.