-28=-7x(3x+4)+21x

Solution for -28=-7x(3x+4)+21x equation:

-28=-7x(3x+4)+21x

We move all terms to the left:

-28-(-7x(3x+4)+21x)=0


We calculate terms in parentheses: -(-7x(3x+4)+21x), so:

-7x(3x+4)+21x

We add all the numbers together, and all the variables

21x-7x(3x+4)

We multiply parentheses

-21x^2+21x-28x

We add all the numbers together, and all the variables

-21x^2-7x

Back to the equation:

-(-21x^2-7x)


We get rid of parentheses

21x^2+7x-28=0

a = 21; b = 7; c = -28;
Δ = b2-4ac
Δ = 72-4·21·(-28)
Δ = 2401
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{2401}=49$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(7)-49}{2*21}=\frac{-56}{42}
=-1+1/3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(7)+49}{2*21}=\frac{42}{42}
=1
$