78=0.4x^2

Solution for 78=0.4x^2 equation:

78=0.4x^2

We move all terms to the left:

78-(0.4x^2)=0

We get rid of parentheses

-0.4x^2+78=0

a = -0.4; b = 0; c = +78;
Δ = b2-4ac
Δ = 02-4·(-0.4)·78
Δ = 124.8
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)-\sqrt{124.8}}{2*-0.4}=\frac{0-\sqrt{124.8}}{-0.8}
=-\frac{\sqrt{}}{-0.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{124.8}}{2*-0.4}=\frac{0+\sqrt{124.8}}{-0.8}
=\frac{\sqrt{}}{-0.8}
$