1.
The value of aa for which the lines x−21=y−92=z−133\displaystyle\frac{x-2}{1}=\frac{y-9}{2}=\frac{z-13}{3} and x−a−1=y−72=z+2−3\displaystyle\frac{x-a}{-1}=\frac{y-7}{2}=\frac{z+2}{-3} intersect is