Answer
$s=-5,-4$
Work Step by Step
$s^2+9s+20=0$
or, $s^2+9s=-20$
Compare it with the standard form of quadratic equation $ax^2+bx+c$, we have $a=1, b=9$
To complete the square, add $\dfrac{81}{4}$ on both sides.
$s^2+9s+\dfrac{81}{4}=-20+\dfrac{81}{4}$
$\implies (s+\dfrac{9}{2})^2=\dfrac{1}{4}$
$\implies (s+4.5)^2=0.25$
$\implies (s+4.5)=0.5$
and
$\implies (s+4.5)=-0.5$
or, $s=-5,-4$
