Question 118355
80=-5t^2-20t+100
{{{-5t^2-20t+100-80)}}}=0 [subtract 80 from both sides]
{{{-5t^2-20t+20)}}}=0 [simplify]

.

{{{x = (-(-20) +- sqrt( (-20)^2-(4*-5*20 )))/(2*a) }}} 
.
{{{x = (20) +- sqrt( ((400)-(-400) ))/(-10))) }}}
.
{{{x = (20) +- sqrt(( (800) ))/(-10))) }}}
.
x=0.828
and
x=-4.828