84=3x(3x+4x)

Solution for 84=3x(3x+4x) equation:

84=3x(3x+4x)

We move all terms to the left:

84-(3x(3x+4x))=0

We add all the numbers together, and all the variables

-(3x(+7x))+84=0


We calculate terms in parentheses: -(3x(+7x)), so:

3x(+7x)

We multiply parentheses

21x^2

Back to the equation:

-(21x^2)


a = -21; b = 0; c = +84;
Δ = b2-4ac
Δ = 02-4·(-21)·84
Δ = 7056
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{7056}=84$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-84}{2*-21}=\frac{-84}{-42}
=+2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+84}{2*-21}=\frac{84}{-42}
=-2
$