-90=-16t2

Solution for -90=-16t2 equation:

-90=-16t^2

We move all terms to the left:

-90-(-16t^2)=0

We get rid of parentheses

16t^2-90=0

a = 16; b = 0; c = -90;
Δ = b2-4ac
Δ = 02-4·16·(-90)
Δ = 5760
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{5760}=\sqrt{576*10}=\sqrt{576}*\sqrt{10}=24\sqrt{10}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-24\sqrt{10}}{2*16}=\frac{0-24\sqrt{10}}{32}
=-\frac{24\sqrt{10}}{32}
=-\frac{3\sqrt{10}}{4}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+24\sqrt{10}}{2*16}=\frac{0+24\sqrt{10}}{32}
=\frac{24\sqrt{10}}{32}
=\frac{3\sqrt{10}}{4}
$