3x(3x+4x)=21

Solution for 3x(3x+4x)=21 equation:

3x(3x+4x)=21

We move all terms to the left:

3x(3x+4x)-(21)=0

We add all the numbers together, and all the variables

3x(+7x)-21=0

We multiply parentheses

21x^2-21=0

a = 21; b = 0; c = -21;
Δ = b2-4ac
Δ = 02-4·21·(-21)
Δ = 1764
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1764}=42$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-42}{2*21}=\frac{-42}{42}
=-1
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+42}{2*21}=\frac{42}{42}
=1
$