Find the distances between planes.
plane 1: 2x-y-z-1=0
plane 2: 2x-y-z-4=0
First pick any point on the second plane. Let's choose
y=0, z=0,
2x = 4
x = 2
(2,0,0) is on plane 2.
Then use the formula for the distance between that point and Plane 1.
You can find that formula at
http://www.math.umn.edu/~nykamp/m2374/readings/planedist/#eq:distformula2