-10.5=7.5t-4.9t^2

Solution for -10.5=7.5t-4.9t^2 equation:

-10.5=7.5t-4.9t^2

We move all terms to the left:

-10.5-(7.5t-4.9t^2)=0

We get rid of parentheses

4.9t^2-7.5t-10.5=0

a = 4.9; b = -7.5; c = -10.5;
Δ = b2-4ac
Δ = -7.52-4·4.9·(-10.5)
Δ = 262.05
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{-(-7.5)-\sqrt{262.05}}{2*4.9}=\frac{7.5-\sqrt{262.05}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-7.5)+\sqrt{262.05}}{2*4.9}=\frac{7.5+\sqrt{262.05}}{9.8}
$