70=-16.4t^2+80t

Solution for 70=-16.4t^2+80t equation:

70=-16.4t^2+80t

We move all terms to the left:

70-(-16.4t^2+80t)=0

We get rid of parentheses

16.4t^2-80t+70=0

a = 16.4; b = -80; c = +70;
Δ = b2-4ac
Δ = -802-4·16.4·70
Δ = 1808
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{1808}=\sqrt{16*113}=\sqrt{16}*\sqrt{113}=4\sqrt{113}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-80)-4\sqrt{113}}{2*16.4}=\frac{80-4\sqrt{113}}{32.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-80)+4\sqrt{113}}{2*16.4}=\frac{80+4\sqrt{113}}{32.8}
$