-5x(-4x+6)=21

Solution for -5x(-4x+6)=21 equation:

-5x(-4x+6)=21

We move all terms to the left:

-5x(-4x+6)-(21)=0

We multiply parentheses

20x^2-30x-21=0

a = 20; b = -30; c = -21;
Δ = b2-4ac
Δ = -302-4·20·(-21)
Δ = 2580
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{2580}=\sqrt{4*645}=\sqrt{4}*\sqrt{645}=2\sqrt{645}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-30)-2\sqrt{645}}{2*20}=\frac{30-2\sqrt{645}}{40}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-30)+2\sqrt{645}}{2*20}=\frac{30+2\sqrt{645}}{40}
$