Answer
$\dfrac{s^2}{2}$
Work Step by Step
RECALL:
(i) $a^m \cdot a^n = a^{m+n}$
(ii) $a^{-m} = \dfrac{1}{a^m}, a\ne0$
Use the rules above to have:
$=\dfrac{5}{10} \cdot s^{-7-(-9)}
\\=\dfrac{1}{2} \cdot s^{-7+9}
\\=\dfrac{1}{2} \cdot s^{2}
\\=\dfrac{s^2}{2}$
