7x(4x+5)=0

Solution for 7x(4x+5)=0 equation:

7x(4x+5)=0

We multiply parentheses

28x^2+35x=0

a = 28; b = 35; c = 0;
Δ = b2-4ac
Δ = 352-4·28·0
Δ = 1225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1225}=35$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(35)-35}{2*28}=\frac{-70}{56}
=-1+1/4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(35)+35}{2*28}=\frac{0}{56}
=0
$