0=5t^2-15t-20

Solution for 0=5t^2-15t-20 equation:

0=5t^2-15t-20

We move all terms to the left:

0-(5t^2-15t-20)=0

We add all the numbers together, and all the variables

-(5t^2-15t-20)=0

We get rid of parentheses

-5t^2+15t+20=0

a = -5; b = 15; c = +20;
Δ = b2-4ac
Δ = 152-4·(-5)·20
Δ = 625
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{625}=25$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-25}{2*-5}=\frac{-40}{-10}
=+4
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+25}{2*-5}=\frac{10}{-10}
=-1
$