(7d-8)(d+7)=0

Solution for (7d-8)(d+7)=0 equation:

(7d-8)(d+7)=0

We multiply parentheses ..

(+7d^2+49d-8d-56)=0

We get rid of parentheses

7d^2+49d-8d-56=0

We add all the numbers together, and all the variables

7d^2+41d-56=0

a = 7; b = 41; c = -56;
Δ = b2-4ac
Δ = 412-4·7·(-56)
Δ = 3249
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3249}=57$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(41)-57}{2*7}=\frac{-98}{14}
=-7
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(41)+57}{2*7}=\frac{16}{14}
=1+1/7
$