Whatever value of 'd' makes the lines' slopes the same would be what you are looking for .....same slope lines are parallel and do not intersect.
 Place each line into y = mx + b   form     m is the slope
  
 15y = 10x - 17
 y = 2/3 x - 17/15          slope = 2/3      so  we need a value of d to make slope of second line = 2/3
  
 dx + 12y = 8
 12 y = - dx + 8
    y =  - d/12  + 2/3             we need    -d/12 = 2/3     so d = -8