From Mechanics

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

and

\displaystyle \tan \alpha =\frac{V}{H}

Here

\displaystyle V=-7\ \text{N} and \displaystyle H=-5\ \text{N}.

Substituting the given values

\displaystyle {{F}^{2}}={{\left( -5 \right)}^{2}}+{{\left( \grave{\ }-7 \right)}^{2}}=74

giving

\displaystyle F=8.6\ \text{N}

Also

\displaystyle \tan \alpha =\frac{-7}{-5}=1\textrm{.}4 giving

\displaystyle \alpha =-125\textrm{.}5{}^\circ

Here the solution in the third quadrant has been chosen.