From Mechanics

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

and

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

Here

\displaystyle V=A and \displaystyle H=B.

Substituting the given values

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

Also

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

giving

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