Hey guys.

http://img202.imageshack.us/img202/6186/35018044.png

In the pic you can a function and its plot.
H changes according to a (if a gets bigger, H becomes smaller). I need to find a exactly where this parabolic shape touches the x axis or in other word where H=0, also I need a to be exact, is there a numerical way to do this?

Thanks a lot.