-66=9(-7-8r)72r

Solution for -66=9(-7-8r)72r equation:

-66=9(-7-8r)72r

We move all terms to the left:

-66-(9(-7-8r)72r)=0

We add all the numbers together, and all the variables

-(9(-8r-7)72r)-66=0


We calculate terms in parentheses: -(9(-8r-7)72r), so:

9(-8r-7)72r

We multiply parentheses

-5184r^2-4536r

Back to the equation:

-(-5184r^2-4536r)


We get rid of parentheses

5184r^2+4536r-66=0

a = 5184; b = 4536; c = -66;
Δ = b2-4ac
Δ = 45362-4·5184·(-66)
Δ = 21943872
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{21943872}=\sqrt{5184*4233}=\sqrt{5184}*\sqrt{4233}=72\sqrt{4233}$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(4536)-72\sqrt{4233}}{2*5184}=\frac{-4536-72\sqrt{4233}}{10368}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(4536)+72\sqrt{4233}}{2*5184}=\frac{-4536+72\sqrt{4233}}{10368}
$