30=(4+0.4x)x

Solution for 30=(4+0.4x)x equation:

30=(4+0.4x)x

We move all terms to the left:

30-((4+0.4x)x)=0

We add all the numbers together, and all the variables

-((0.4x+4)x)+30=0


We calculate terms in parentheses: -((0.4x+4)x), so:

(0.4x+4)x

We multiply parentheses

0x^2+4x

We add all the numbers together, and all the variables

x^2+4x

Back to the equation:

-(x^2+4x)


We get rid of parentheses

-x^2-4x+30=0

We add all the numbers together, and all the variables

-1x^2-4x+30=0

a = -1; b = -4; c = +30;
Δ = b2-4ac
Δ = -42-4·(-1)·30
Δ = 136
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{136}=\sqrt{4*34}=\sqrt{4}*\sqrt{34}=2\sqrt{34}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-4)-2\sqrt{34}}{2*-1}=\frac{4-2\sqrt{34}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4)+2\sqrt{34}}{2*-1}=\frac{4+2\sqrt{34}}{-2}
$