6.  If the lines  and 
are perpendicular, find the value of k.
                
                  Given both lines are perpendicular so we have the relation; 
For the two lines whose direction ratios are known,
 
We have the direction ratios of the lines,   and  
  are 
  and 
 respectively.
Therefore applying the formula,
  or  
 For, 
 the lines are perpendicular.