(9-x)(7x+4)=0

Solution for (9-x)(7x+4)=0 equation:

(9-x)(7x+4)=0

We add all the numbers together, and all the variables

(-1x+9)(7x+4)=0

We multiply parentheses ..

(-7x^2-4x+63x+36)=0

We get rid of parentheses

-7x^2-4x+63x+36=0

We add all the numbers together, and all the variables

-7x^2+59x+36=0

a = -7; b = 59; c = +36;
Δ = b2-4ac
Δ = 592-4·(-7)·36
Δ = 4489
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{4489}=67$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(59)-67}{2*-7}=\frac{-126}{-14}
=+9
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(59)+67}{2*-7}=\frac{8}{-14}
=-4/7
$