90=-8t2+60t

Solution for 90=-8t2+60t equation:

90=-8t^2+60t

We move all terms to the left:

90-(-8t^2+60t)=0

We get rid of parentheses

8t^2-60t+90=0

a = 8; b = -60; c = +90;
Δ = b2-4ac
Δ = -602-4·8·90
Δ = 720
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:

$\sqrt{\Delta}=\sqrt{720}=\sqrt{144*5}=\sqrt{144}*\sqrt{5}=12\sqrt{5}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-60)-12\sqrt{5}}{2*8}=\frac{60-12\sqrt{5}}{16}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-60)+12\sqrt{5}}{2*8}=\frac{60+12\sqrt{5}}{16}
$