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