15=20t-1.86t^2

Solution for 15=20t-1.86t^2 equation:

15=20t-1.86t^2

We move all terms to the left:

15-(20t-1.86t^2)=0

We get rid of parentheses

1.86t^2-20t+15=0

a = 1.86; b = -20; c = +15;
Δ = b2-4ac
Δ = -202-4·1.86·15
Δ = 288.4
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{-(-20)-\sqrt{288.4}}{2*1.86}=\frac{20-\sqrt{288.4}}{3.72}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+\sqrt{288.4}}{2*1.86}=\frac{20+\sqrt{288.4}}{3.72}
$