21=w^2+2.5w

Solution for 21=w^2+2.5w equation:

21=w^2+2.5w

We move all terms to the left:

21-(w^2+2.5w)=0

We get rid of parentheses

-w^2-2.5w+21=0

We add all the numbers together, and all the variables

-1w^2-2.5w+21=0

a = -1; b = -2.5; c = +21;
Δ = b2-4ac
Δ = -2.52-4·(-1)·21
Δ = 90.25
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$w_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$w_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$w_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2.5)-\sqrt{90.25}}{2*-1}=\frac{2.5-\sqrt{90.25}}{-2}
$
$w_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2.5)+\sqrt{90.25}}{2*-1}=\frac{2.5+\sqrt{90.25}}{-2}
$