2x^2-6x=21

Solution for 2x^2-6x=21 equation:

2x^2-6x=21

We move all terms to the left:

2x^2-6x-(21)=0

a = 2; b = -6; c = -21;
Δ = b2-4ac
Δ = -62-4·2·(-21)
Δ = 204
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:

$\sqrt{\Delta}=\sqrt{204}=\sqrt{4*51}=\sqrt{4}*\sqrt{51}=2\sqrt{51}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-6)-2\sqrt{51}}{2*2}=\frac{6-2\sqrt{51}}{4}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-6)+2\sqrt{51}}{2*2}=\frac{6+2\sqrt{51}}{4}
$