30=(5+0.4x)x

Solution for 30=(5+0.4x)x equation:

30=(5+0.4x)x

We move all terms to the left:

30-((5+0.4x)x)=0

We add all the numbers together, and all the variables

-((0.4x+5)x)+30=0


We calculate terms in parentheses: -((0.4x+5)x), so:

(0.4x+5)x

We multiply parentheses

0x^2+5x

We add all the numbers together, and all the variables

x^2+5x

Back to the equation:

-(x^2+5x)


We get rid of parentheses

-x^2-5x+30=0

We add all the numbers together, and all the variables

-1x^2-5x+30=0

a = -1; b = -5; c = +30;
Δ = b2-4ac
Δ = -52-4·(-1)·30
Δ = 145
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{-(-5)-\sqrt{145}}{2*-1}=\frac{5-\sqrt{145}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-5)+\sqrt{145}}{2*-1}=\frac{5+\sqrt{145}}{-2}
$