72=-4.9t^2+68t

Solution for 72=-4.9t^2+68t equation:

72=-4.9t^2+68t

We move all terms to the left:

72-(-4.9t^2+68t)=0

We get rid of parentheses

4.9t^2-68t+72=0

a = 4.9; b = -68; c = +72;
Δ = b2-4ac
Δ = -682-4·4.9·72
Δ = 3212.8
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-68)-\sqrt{3212.8}}{2*4.9}=\frac{68-\sqrt{3212.8}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-68)+\sqrt{3212.8}}{2*4.9}=\frac{68+\sqrt{3212.8}}{9.8}
$