From Mechanics

The diagram shows the forces acting on the ladder.

Taking moments about the base of the ladder:

\displaystyle \begin{align} & 5\sin \theta \times S=2\textrm{.}5\cos \theta \times 196 \\ & S=\frac{2\textrm{.}5\cos \theta \times 196}{5\sin \theta }=\frac{98}{\tan \theta }\text{ N} \\ \end{align}