Calculate normal vector to this plane :
N = s x t
(vector product of two vectors belonging to plane)
Now you have coefficients a, b, c:
N = (a, b, c)
then substitute base point (in general - any point in the plane)
(1, 2, -1) to equation ax+yb+cz+d=0
a+2b-c+d=0
and find d