Greetings from Brasil...
Just take the cross product of the vectors EF and EH. The result will be a vector n. This vector (n) will help in the equation of the plane
vector EF = point F - point E
vector EH = point H - point E
check the attachment
according to the example in the annex, we notice that the points E, F and G are in the plane whose equation will be:
π : 6X + 9Y + 5Z + 1 = 0