A=9h-2.25h^2

Solution for A=9h-2.25h^2 equation:

=9A-2.25A^2

We move all terms to the left:

-(9A-2.25A^2)=0

We get rid of parentheses

2.25A^2-9A=0

a = 2.25; b = -9; c = 0;
Δ = b2-4ac
Δ = -92-4·2.25·0
Δ = 81
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{81}=9$
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-9}{2*2.25}=\frac{0}{4.5}
=0
$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+9}{2*2.25}=\frac{18}{4.5}
=4
$