Answer:
x + y + z = 14
Step-by-step explanation:
If the points are designated A, B, C, then ...
AB × AC = (7, -7, 0) × (7, 0, -7) = (49, 49, 49).
That is, a vector perpendicular to the plane is (1, 1, 1), so the equation of the plane can be ...
(1, 1, 1)·(x, y, z) = (1, 1, 1)·A
x + y + z = 14
