-21x^2-16x+16=0

Solution for -21x^2-16x+16=0 equation:

-21x^2-16x+16=0

a = -21; b = -16; c = +16;
Δ = b2-4ac
Δ = -162-4·(-21)·16
Δ = 1600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1600}=40$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-16)-40}{2*-21}=\frac{-24}{-42}
=4/7
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-16)+40}{2*-21}=\frac{56}{-42}
=-1+1/3
$