21=-16t^2+88t

Solution for 21=-16t^2+88t equation:

21=-16t^2+88t

We move all terms to the left:

21-(-16t^2+88t)=0

We get rid of parentheses

16t^2-88t+21=0

a = 16; b = -88; c = +21;
Δ = b2-4ac
Δ = -882-4·16·21
Δ = 6400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{6400}=80$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-88)-80}{2*16}=\frac{8}{32}
=1/4
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-88)+80}{2*16}=\frac{168}{32}
=5+1/4
$