Eliminate the variables s and t in the system of simultaneous equations:

[1]: x = 2 - 3s + t

[2]: y = 4 + 0s + 0t

[3]: z = 1 + s + t

Calculate s and t using [1] and [3] and plug the results into [2]

If you now feel as if someone has made a practical joke you can use the cross-product of to determine the equation of the plane.