Question 706862
That is an equation already solved for h.  What are you looking for?  It appears to be a quadratic equation with the variable, t.  You can solve for t.  You would treat just t as the only variable, and put the equation into the typical, "=0" form, and use general solution for a quadratic equation:


{{{ h=-81t^2+at+c }}}
{{{-81t^2+at+c-h=0}}}
{{{-81t^2+at+(c-h)=0}}}


Your solutions for t are these:
{{{t=(-a+sqrt(a^2-4*(-81)*(c-h)))/(2(-81))}}}
OR
{{{t=(-a-sqrt(a^2-4*(-81)*(c-h)))/(2(-81))}}}


You might then take, for example the "+" form, as the simpler looking:
{{{(-a+sqrt(a^2+324(c-h)))/(-162)}}}
and the "-" form, as:
{{{(-a-sqrt(a^2+324(c-h)))/(-162)}}}