7x^2-20x-21=0

Solution for 7x^2-20x-21=0 equation:

7x^2-20x-21=0

a = 7; b = -20; c = -21;
Δ = b2-4ac
Δ = -202-4·7·(-21)
Δ = 988
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:

$\sqrt{\Delta}=\sqrt{988}=\sqrt{4*247}=\sqrt{4}*\sqrt{247}=2\sqrt{247}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-2\sqrt{247}}{2*7}=\frac{20-2\sqrt{247}}{14}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+2\sqrt{247}}{2*7}=\frac{20+2\sqrt{247}}{14}
$