-0.015x^2+0.64x+1.6=6.5

Solution for -0.015x^2+0.64x+1.6=6.5 equation:

-0.015x^2+0.64x+1.6=6.5

We move all terms to the left:

-0.015x^2+0.64x+1.6-(6.5)=0

We add all the numbers together, and all the variables

-0.015x^2+0.64x-4.9=0

a = -0.015; b = 0.64; c = -4.9;
Δ = b2-4ac
Δ = 0.642-4·(-0.015)·(-4.9)
Δ = 0.1156
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0.64)-\sqrt{0.1156}}{2*-0.015}=\frac{-0.64-\sqrt{0.1156}}{-0.03}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0.64)+\sqrt{0.1156}}{2*-0.015}=\frac{-0.64+\sqrt{0.1156}}{-0.03}
$