The midpoint formula in 3d is: #((x_1+x_2)/2, (y_1+y_2)/2, (z_1+z_2)/2)#, which in this case is #((-1+0)/2, (2+6)/2, (4+(-3))/2)=(-1/2, 4, 1/2)#. This makes sense, since the #x, y, and z# coordinates are halfway between those of the given points in each case.