You want the (positive) distance when the height is 0, 0= -d^2 + 10d +5.
You would factor that out(You'd get radicals), and the final answer would be
<span>5±<span>30<span>−−</span>√</span></span><span>
You'd take the only positive one of the two, 5 + srt(30)
I hope I did that right :/</span>