0.65=7.625t-4.905t^2

Solution for 0.65=7.625t-4.905t^2 equation:

0.65=7.625t-4.905t^2

We move all terms to the left:

0.65-(7.625t-4.905t^2)=0

We get rid of parentheses

4.905t^2-7.625t+0.65=0

a = 4.905; b = -7.625; c = +0.65;
Δ = b2-4ac
Δ = -7.6252-4·4.905·0.65
Δ = 45.387625
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{-(-7.625)-\sqrt{45.387625}}{2*4.905}=\frac{7.625-\sqrt{45.387625}}{9.81}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-7.625)+\sqrt{45.387625}}{2*4.905}=\frac{7.625+\sqrt{45.387625}}{9.81}
$