input: Write a function to calculate the distance between two points in 3D space.
  assert distance(3, 4, 9, -5, -6, -7) == 18.16590212458495
  assert distance(1, 2, 3, 4, 5, 6) == 5.196152422706632
  assert distance(0, 0, 0, 0, 0, 0) == 0
output: def distance(x1, y1, z1, x2, y2, z2):
      return ((x2 - x1)**2 + (y2 - y1)**2 + (z2 - z1)**2)**0.5