From Mechanics

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

and

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

Here

\displaystyle V=-3\ \text{N} and \displaystyle H=4\ \text{N}.

Substituting the given values

\displaystyle {{F}^{2}}={{4}^{2}}+{{\left( -3 \right)}^{2}}=25

giving

\displaystyle F=5\ \text{N}

Also

\displaystyle \tan \alpha =\frac{-3}{4}

giving

\displaystyle \alpha =-36\textrm{.}9{}^\circ