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 \begin{align} {{F}^{2}}={{4}^{2}}+{{3}^{2}}=25 \\ \end{align}

giving

\displaystyle \begin{align} F=5\ \text{N} \\ \end{align}

Also

\displaystyle \tan \alpha =\frac{3}{4}=0\textrm{.}75

giving

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