Step 1 :1) \( D(A_0, \vec{u}_0) : \begin{cases} x = 1 + 2t \\ y = 1 - \frac{1}{2}t \\ z = 1 + 2t \end{cases} \)
Step 2 :2) \( P(A, \vec{u}, \vec{v}): (x + 3)y - z + 3 = 0 \)
Step 3 :3) \( I(3, 0, 3) \) is the intersection point of D and P.
Step 4 :4) Cartesian equations of D: \( \frac{x-1}{2} = y-1 = \frac{z-1}{2} \)