2(x^2-2x-1.25)=0

Solution for 2(x^2-2x-1.25)=0 equation:

2(x^2-2x-1.25)=0

We multiply parentheses

2x^2-4x-2.5=0

a = 2; b = -4; c = -2.5;
Δ = b2-4ac
Δ = -42-4·2·(-2.5)
Δ = 36
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{36}=6$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-4)-6}{2*2}=\frac{-2}{4}
=-1/2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4)+6}{2*2}=\frac{10}{4}
=2+1/2
$