15=-1,25x^2+10x

Solution for 15=-1,25x^2+10x equation:

15=-1.25x^2+10x

We move all terms to the left:

15-(-1.25x^2+10x)=0

We get rid of parentheses

1.25x^2-10x+15=0

a = 1.25; b = -10; c = +15;
Δ = b2-4ac
Δ = -102-4·1.25·15
Δ = 25
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{25}=5$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-10)-5}{2*1.25}=\frac{5}{2.5}
=2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-10)+5}{2*1.25}=\frac{15}{2.5}
=6
$