-4(u+3)(-u+7)=0

Solution for -4(u+3)(-u+7)=0 equation:

-4(u+3)(-u+7)=0

We add all the numbers together, and all the variables

-4(u+3)(-1u+7)=0

We multiply parentheses ..

-4(-1u^2+7u-3u+21)=0

We multiply parentheses

4u^2-28u+12u-84=0

We add all the numbers together, and all the variables

4u^2-16u-84=0

a = 4; b = -16; c = -84;
Δ = b2-4ac
Δ = -162-4·4·(-84)
Δ = 1600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1600}=40$
$u_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-16)-40}{2*4}=\frac{-24}{8}
=-3
$
$u_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-16)+40}{2*4}=\frac{56}{8}
=7
$