(4c+2)(7c-21)=0

Solution for (4c+2)(7c-21)=0 equation:

(4c+2)(7c-21)=0

We multiply parentheses ..

(+28c^2-84c+14c-42)=0

We get rid of parentheses

28c^2-84c+14c-42=0

We add all the numbers together, and all the variables

28c^2-70c-42=0

a = 28; b = -70; c = -42;
Δ = b2-4ac
Δ = -702-4·28·(-42)
Δ = 9604
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{9604}=98$
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-70)-98}{2*28}=\frac{-28}{56}
=-1/2
$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-70)+98}{2*28}=\frac{168}{56}
=3
$