3x2-9x-21=0

Solution for 3x2-9x-21=0 equation:

3x^2-9x-21=0

a = 3; b = -9; c = -21;
Δ = b2-4ac
Δ = -92-4·3·(-21)
Δ = 333
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:

$\sqrt{\Delta}=\sqrt{333}=\sqrt{9*37}=\sqrt{9}*\sqrt{37}=3\sqrt{37}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-3\sqrt{37}}{2*3}=\frac{9-3\sqrt{37}}{6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+3\sqrt{37}}{2*3}=\frac{9+3\sqrt{37}}{6}
$