With your question there are two different ways to find the answer:

1. Choose any arbitrary point on the 1rst line; determine the equation of a line perpendicular to the 1rst line passing through this point; calculate the intersection point of the perpendicular line and the 2nd line; calculate the distance between the choosen point and the intersection point.

2. The equations of the two lines are given in normal form:

with

indicating the perpendicular direction to the line and

12 indicating the perpendicular distance of the origin to the line.

If the normal vector is a unit vector then the distance of origin to the line is measured in units too. The difference between these two distances must be the distance between the two parallels:

Therefore the equation of the two lines become:

Therefore the distance between the parallels is: