What would the formula be for finding the coords of a second triangle

I have a triangle being drawn, its coordinates are A(X, Y, Z), B(X, Y, Z), and C(X, Y, Z)

I need to draw another triangle that is basically the same triangle that is originally being drawn mirrored over itself. Two of the coordinates would be the same because they lie on top of each other, however I do not know the formula for finding the coordinates of the third vertex of the second triangle. If you can help me I will send you 500,000,000,000,000,000 yen!

These two triangles combined will form a solid quadrilateral.