AB
Write the coordinates in the boxes.  Then press 

xA = yA = zA =
xB = yB = zB =

d(A,B) =

xM =    yM =    zM =

d(A,B)^2 =

line AB:  x = xA+(xB-xA)*t, y = yA+(yB-yA)*t, z = zA+(zB-zA)*t

If you have to put a coefficient using some function symbol (for example 1/5, √2), put it in a numerical form (0.2, 1.414213562373095), for example using this (or this)
examples