n:=cross(A-C,B-C)/9
n=(-3,3,-1)
E1:= n ((x,y,z) - C)=0 => n*(x,y,z) - n*C =0 => n(x,y,z) + 3 =0
\(E1: \, -3 \; x + 3 \; y - z + 3 = 0\)
g(t):=P+t (Q-P)
\(g(t) \, := \, \left(1 + 4 \; t, 1 + 2 \; t, 1 - 2 \; t \right) \)
g∈E1: -3 (1 + 4 * t) + 3 (1 + 2 * t) - (1 - 2 * t) + 3 =0
\(-4 \; t + 2\) => t
t∈g = > \(S \, := \, \left(3, 2, 0 \right)\)