36-21x-3x^2=0

Solution for 36-21x-3x^2=0 equation:

36-21x-3x^2=0

a = -3; b = -21; c = +36;
Δ = b2-4ac
Δ = -212-4·(-3)·36
Δ = 873
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:

$\sqrt{\Delta}=\sqrt{873}=\sqrt{9*97}=\sqrt{9}*\sqrt{97}=3\sqrt{97}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-3\sqrt{97}}{2*-3}=\frac{21-3\sqrt{97}}{-6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+3\sqrt{97}}{2*-3}=\frac{21+3\sqrt{97}}{-6}
$