12-3x(2x+5)=21

Solution for 12-3x(2x+5)=21 equation:

12-3x(2x+5)=21

We move all terms to the left:

12-3x(2x+5)-(21)=0

We add all the numbers together, and all the variables

-3x(2x+5)-9=0

We multiply parentheses

-6x^2-15x-9=0

a = -6; b = -15; c = -9;
Δ = b2-4ac
Δ = -152-4·(-6)·(-9)
Δ = 9
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{9}=3$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-3}{2*-6}=\frac{12}{-12}
=-1
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+3}{2*-6}=\frac{18}{-12}
=-1+1/2
$