15r^2+8=21r+8

Solution for 15r^2+8=21r+8 equation:

15r^2+8=21r+8

We move all terms to the left:

15r^2+8-(21r+8)=0

We get rid of parentheses

15r^2-21r-8+8=0

We add all the numbers together, and all the variables

15r^2-21r=0

a = 15; b = -21; c = 0;
Δ = b2-4ac
Δ = -212-4·15·0
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-21}{2*15}=\frac{0}{30}
=0
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+21}{2*15}=\frac{42}{30}
=1+2/5
$