(3-z)(5z/7)=0

Solution for (3-z)(5z/7)=0 equation:

(3-z)(5z/7)=0

We add all the numbers together, and all the variables

(-1z+3)(+5z/7)=0

We multiply parentheses ..

(-5z^2+15z)=0

We get rid of parentheses

-5z^2+15z=0

a = -5; b = 15; c = 0;
Δ = b2-4ac
Δ = 152-4·(-5)·0
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-15}{2*-5}=\frac{-30}{-10}
=+3
$
$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+15}{2*-5}=\frac{0}{-10}
=0
$