75=0.05r^2+r

Solution for 75=0.05r^2+r equation:

75=0.05r^2+r

We move all terms to the left:

75-(0.05r^2+r)=0

We get rid of parentheses

-0.05r^2-r+75=0

We add all the numbers together, and all the variables

-0.05r^2-1r+75=0

a = -0.05; b = -1; c = +75;
Δ = b2-4ac
Δ = -12-4·(-0.05)·75
Δ = 16
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{16}=4$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1)-4}{2*-0.05}=\frac{-3}{-0.1}
=+30
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+4}{2*-0.05}=\frac{5}{-0.1}
=-50
$