E: X = [2, 3, 1] + r·[5, 1, 2] + s·[3, 1, 0]
N = [5, 1, 2] ⨯ [3, 1, 0] = [-2, 6, 2] = - 2·[1, -3, -1]
E: X·[1, -3, -1] = [2, 3, 1]·[1, -3, -1]
E: x - 3·y - z = -8
d = (x - 3·y - z + 8)/√(1^2 + 3^2 + 1^2)
d = ((2) - 3·(7) - (-9) + 8)/√(1^2 + 3^2 + 1^2) = - 2·√11/11 = -0.6030