(x+2.5)(x+7.8)=0

Solution for (x+2.5)(x+7.8)=0 equation:

(x+2.5)(x+7.8)=0

We multiply parentheses ..

(+x^2+7.8x+2.5x+19.5)=0

We get rid of parentheses

x^2+7.8x+2.5x+19.5=0

We add all the numbers together, and all the variables

x^2+10.3x+19.5=0

a = 1; b = 10.3; c = +19.5;
Δ = b2-4ac
Δ = 10.32-4·1·19.5
Δ = 28.09
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{-(10.3)-\sqrt{28.09}}{2*1}=\frac{-10.3-\sqrt{28.09}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(10.3)+\sqrt{28.09}}{2*1}=\frac{-10.3+\sqrt{28.09}}{2}
$