21=4s+s2

Solution for 21=4s+s2 equation:

21=4s+s2

We move all terms to the left:

21-(4s+s2)=0

We add all the numbers together, and all the variables

-(+4s+s^2)+21=0

We get rid of parentheses

-s^2-4s+21=0

We add all the numbers together, and all the variables

-1s^2-4s+21=0

a = -1; b = -4; c = +21;
Δ = b2-4ac
Δ = -42-4·(-1)·21
Δ = 100
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$s_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$s_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{100}=10$
$s_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-4)-10}{2*-1}=\frac{-6}{-2}
=+3
$
$s_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4)+10}{2*-1}=\frac{14}{-2}
=-7
$