-1.2=-4.9t^2+21t

Solution for -1.2=-4.9t^2+21t equation:

-1.2=-4.9t^2+21t

We move all terms to the left:

-1.2-(-4.9t^2+21t)=0

We get rid of parentheses

4.9t^2-21t-1.2=0

a = 4.9; b = -21; c = -1.2;
Δ = b2-4ac
Δ = -212-4·4.9·(-1.2)
Δ = 464.52
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{-(-21)-\sqrt{464.52}}{2*4.9}=\frac{21-\sqrt{464.52}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+\sqrt{464.52}}{2*4.9}=\frac{21+\sqrt{464.52}}{9.8}
$