0=2.1+9.3t-4.905t^2

Solution for 0=2.1+9.3t-4.905t^2 equation:

0=2.1+9.3t-4.905t^2

We move all terms to the left:

0-(2.1+9.3t-4.905t^2)=0

We add all the numbers together, and all the variables

-(2.1+9.3t-4.905t^2)=0

We get rid of parentheses

4.905t^2-9.3t-2.1=0

a = 4.905; b = -9.3; c = -2.1;
Δ = b2-4ac
Δ = -9.32-4·4.905·(-2.1)
Δ = 127.692
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{-(-9.3)-\sqrt{127.692}}{2*4.905}=\frac{9.3-\sqrt{127.692}}{9.81}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9.3)+\sqrt{127.692}}{2*4.905}=\frac{9.3+\sqrt{127.692}}{9.81}
$