15=-4.9t^2+18.5t

Solution for 15=-4.9t^2+18.5t equation:

15=-4.9t^2+18.5t

We move all terms to the left:

15-(-4.9t^2+18.5t)=0

We get rid of parentheses

4.9t^2-18.5t+15=0

a = 4.9; b = -18.5; c = +15;
Δ = b2-4ac
Δ = -18.52-4·4.9·15
Δ = 48.25
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{-(-18.5)-\sqrt{48.25}}{2*4.9}=\frac{18.5-\sqrt{48.25}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-18.5)+\sqrt{48.25}}{2*4.9}=\frac{18.5+\sqrt{48.25}}{9.8}
$