From Mechanics

Using \displaystyle {{F}^{2}}={{H}^{2}}+{{V}^{2}}

Substituting the given values

\displaystyle \begin{align} & {{F}^{2}}={{9}^{2}}+{ {{\left( -10 \right)}^{2}} }=181 \\ & F=13\textrm{.}5\ \text{N} \\ \end{align}