From Mechanics

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

Substituting the given values

\displaystyle \begin{align} & {{F}^{2}}={{4}^{2}}+{ {3}^{2}} =25 \\ & F=5 \text{ N} \\ \end{align}

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

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

Thus

\displaystyle \tan \alpha =\frac{3}{4}=0\textrm{.}75 giving \displaystyle \alpha =36\textrm{.}9{}^\circ