31=8.7x-0.3x^2

Solution for 31=8.7x-0.3x^2 equation:

31=8.7x-0.3x^2

We move all terms to the left:

31-(8.7x-0.3x^2)=0

We get rid of parentheses

0.3x^2-8.7x+31=0

a = 0.3; b = -8.7; c = +31;
Δ = b2-4ac
Δ = -8.72-4·0.3·31
Δ = 38.49
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8.7)-\sqrt{38.49}}{2*0.3}=\frac{8.7-\sqrt{38.49}}{0.6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8.7)+\sqrt{38.49}}{2*0.3}=\frac{8.7+\sqrt{38.49}}{0.6}
$