840000=0.638x2+6.671x+627.619

Solution for 840000=0.638x2+6.671x+627.619 equation:

840000=0.638x^2+6.671x+627.619

We move all terms to the left:

840000-(0.638x^2+6.671x+627.619)=0

We get rid of parentheses

-0.638x^2-6.671x-627.619+840000=0

We add all the numbers together, and all the variables

-0.638x^2-6.671x+839372.381=0

a = -0.638; b = -6.671; c = +839372.381;
Δ = b2-4ac
Δ = -6.6712-4·(-0.638)·839372.381
Δ = 2142122.818553
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{-(-6.671)-\sqrt{2142122.818553}}{2*-0.638}=\frac{6.671-\sqrt{2142122.818553}}{-1.276}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-6.671)+\sqrt{2142122.818553}}{2*-0.638}=\frac{6.671+\sqrt{2142122.818553}}{-1.276}
$