-3b(2b-7)=-3b+21-3b

Solution for -3b(2b-7)=-3b+21-3b equation:

-3b(2b-7)=-3b+21-3b

We move all terms to the left:

-3b(2b-7)-(-3b+21-3b)=0

We add all the numbers together, and all the variables

-3b(2b-7)-(-6b+21)=0

We multiply parentheses

-6b^2+21b-(-6b+21)=0

We get rid of parentheses

-6b^2+21b+6b-21=0

We add all the numbers together, and all the variables

-6b^2+27b-21=0

a = -6; b = 27; c = -21;
Δ = b2-4ac
Δ = 272-4·(-6)·(-21)
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(27)-15}{2*-6}=\frac{-42}{-12}
=3+1/2
$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(27)+15}{2*-6}=\frac{-12}{-12}
=1
$