15x^2-21x-72=0

Solution for 15x^2-21x-72=0 equation:

15x^2-21x-72=0

a = 15; b = -21; c = -72;
Δ = b2-4ac
Δ = -212-4·15·(-72)
Δ = 4761
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{4761}=69$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-69}{2*15}=\frac{-48}{30}
=-1+3/5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+69}{2*15}=\frac{90}{30}
=3
$