I=90t-5t^2

Solution for I=90t-5t^2 equation:

=90I-5I^2

We move all terms to the left:

-(90I-5I^2)=0

We get rid of parentheses

5I^2-90I=0

a = 5; b = -90; c = 0;
Δ = b2-4ac
Δ = -902-4·5·0
Δ = 8100
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$I_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$I_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{8100}=90$
$I_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-90)-90}{2*5}=\frac{0}{10}
=0
$
$I_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-90)+90}{2*5}=\frac{180}{10}
=18
$