So go ahead and pick a point on the line line as you did: (-7, 6, -1) works fine. But don't just pick a point on the plane at random; find the equation of the line through that point perpendicular to the plane. Since <-5, 8, -7> is the normal vector, x= -7- 5t, y= 6+ 8t, z= -1- 7t is the equation of that line. Find the point where that line intersects the plane. The distance between that point of intersection and (-7, 6, -1) is the distance between the line and plane.