Answer
$$2tx+ty+cz=d.$$
Work Step by Step
Let the equation of any plane that intersects the $ xy $-plane in the line $ r(t)=t\langle 2,1,0\rangle $ be given by
$$ ax+by+cz=d.$$
Now, to find the $ xy $ intersection, we put $ z=0$ and hence we get
$$ ax+ by=d.$$
Due to the intersection, we have
$$ a=2t, \quad b=t .$$
So the equation of any plane whose intersection with the $ xy $-plane is the line $ r(t)=t\langle 2,1,0\rangle $ is given by
$$2tx+ty+cz=d.$$
