15=4.9t^2-1.7t

Solution for 15=4.9t^2-1.7t equation:

15=4.9t^2-1.7t

We move all terms to the left:

15-(4.9t^2-1.7t)=0

We get rid of parentheses

-4.9t^2+1.7t+15=0

a = -4.9; b = 1.7; c = +15;
Δ = b2-4ac
Δ = 1.72-4·(-4.9)·15
Δ = 296.89
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{-(1.7)-\sqrt{296.89}}{2*-4.9}=\frac{-1.7-\sqrt{296.89}}{-9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1.7)+\sqrt{296.89}}{2*-4.9}=\frac{-1.7+\sqrt{296.89}}{-9.8}
$