66=4(.64+.2a)a

Solution for 66=4(.64+.2a)a equation:

66=4(.64+.2a)a

We move all terms to the left:

66-(4(.64+.2a)a)=0

We add all the numbers together, and all the variables

-(4(.2a+0.64)a)+66=0


We calculate terms in parentheses: -(4(.2a+0.64)a), so:

4(.2a+0.64)a

We multiply parentheses

4a^2+2.56a

Back to the equation:

-(4a^2+2.56a)


We get rid of parentheses

-4a^2-2.56a+66=0

a = -4; b = -2.56; c = +66;
Δ = b2-4ac
Δ = -2.562-4·(-4)·66
Δ = 1062.5536
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2.56)-\sqrt{1062.5536}}{2*-4}=\frac{2.56-\sqrt{1062.5536}}{-8}
$
$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2.56)+\sqrt{1062.5536}}{2*-4}=\frac{2.56+\sqrt{1062.5536}}{-8}
$