check the picture below.
now, if we split that triangle like so with that red line, we can use the right-triangle there, to get θ.
[tex]\bf tan(\theta )=\cfrac{\stackrel{opposite}{3}}{\stackrel{adjacent}{3}}\implies tan(\theta )=1\implies tan^{-1}(\theta )=tan^{-1}(1)\implies \measuredangle \theta =\cfrac{\pi }{4}[/tex]
