# How do I convert the polar coordinates 3(cos 210^circ +i\ sin 210^circ) into rectangular form?

You must multiply the modulus, $3$, by the values of the $\cos$ and $\sin$, as:
z=3cos(210°)+i3sin(210°)=
to get the rectangular form: $z = a + i b$
$= - 3 \frac{\sqrt{3}}{2} - i \frac{3}{2}$
cos(210°)=-sqrt(3)/2
sin(210°)=-1/2