r^2-16r=225

Solution for r^2-16r=225 equation:

r^2-16r=225

We move all terms to the left:

r^2-16r-(225)=0

a = 1; b = -16; c = -225;
Δ = b2-4ac
Δ = -162-4·1·(-225)
Δ = 1156
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1156}=34$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-16)-34}{2*1}=\frac{-18}{2}
=-9
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-16)+34}{2*1}=\frac{50}{2}
=25
$