Answer
$5 \sqrt 3$
Work Step by Step
As we are given that, $d=s \sqrt 3$
The formula for $s$ can be defined as:
$s=\dfrac{d}{\sqrt 3}$
Plug $15$ for $d$
Then, we have
$s=\dfrac{15}{\sqrt 3}=\dfrac{5(\sqrt 3)^2}{\sqrt 3}$
or, $s=5 \sqrt 3$