15=2x+0.6x^2

Solution for 15=2x+0.6x^2 equation:

15=2x+0.6x^2

We move all terms to the left:

15-(2x+0.6x^2)=0

We get rid of parentheses

-0.6x^2-2x+15=0

a = -0.6; b = -2; c = +15;
Δ = b2-4ac
Δ = -22-4·(-0.6)·15
Δ = 40
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{40}=\sqrt{4*10}=\sqrt{4}*\sqrt{10}=2\sqrt{10}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2)-2\sqrt{10}}{2*-0.6}=\frac{2-2\sqrt{10}}{-1.2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2)+2\sqrt{10}}{2*-0.6}=\frac{2+2\sqrt{10}}{-1.2}
$