0=-9r^2+21r+26

Solution for 0=-9r^2+21r+26 equation:

0=-9r^2+21r+26

We move all terms to the left:

0-(-9r^2+21r+26)=0

We add all the numbers together, and all the variables

-(-9r^2+21r+26)=0

We get rid of parentheses

9r^2-21r-26=0

a = 9; b = -21; c = -26;
Δ = b2-4ac
Δ = -212-4·9·(-26)
Δ = 1377
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:

$\sqrt{\Delta}=\sqrt{1377}=\sqrt{81*17}=\sqrt{81}*\sqrt{17}=9\sqrt{17}$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-9\sqrt{17}}{2*9}=\frac{21-9\sqrt{17}}{18}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+9\sqrt{17}}{2*9}=\frac{21+9\sqrt{17}}{18}
$