Question: Buttons and Graphs

I am attaching my file. Many of you have given great help on how to get dynamic Maple activities working.  My question again is along those lines. I have created a parabola with sliders a,h, and k.  What I am trying to do is have a button that says show vertex on the equation . However, when I connect the button with Plot3, I plot the the vertex in a new window.  Here is the most successful code I used:

Do(%Plot3=(plot([[%Slider2,%Slider1]],style=point,colour=blue,symbol=solidcircle,symbolsize=18)));

So I do get the point I want (h,k), but the point is not on the function itself.  I have tried muliple maple commands with no luck.  I am wonder how I can get the point (h,k) to be on the function when I hit the "show vertex" button. 

Parabolas_In_Vertex_.mw

Thank you for you time,

Nicholas