$Ok so I tried doing it this way and got $z=2i$ and $w=-1+i\sqrt3\\\\$Now, $z-w=2i-(-1+i\sqrt3)\\\\\indent\indent\indent=1+i(2-\sqrt3)\\\\$To find arg (z-w), we use $tan \theta=\frac{2-\sqrt3}{1}$ where \theta=arg (z-w)\\\\tan\theta=2-\sqrt3\\\\\theta\approx 0.26\\\\\therefore arg (z-w)=0.26