0=-15t+4.9t^2

Solution for 0=-15t+4.9t^2 equation:

0=-15t+4.9t^2

We move all terms to the left:

0-(-15t+4.9t^2)=0

We add all the numbers together, and all the variables

-(-15t+4.9t^2)=0

We get rid of parentheses

-4.9t^2+15t=0

a = -4.9; b = 15; c = 0;
Δ = b2-4ac
Δ = 152-4·(-4.9)·0
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-15}{2*-4.9}=\frac{-30}{-9.8}
=3+0.6/9.8
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+15}{2*-4.9}=\frac{0}{-9.8}
=0
$