In polar form, you express every point with two coordinates [tex](\rho,\theta)[/tex]
The first coordinate [tex]\rho[/tex] represents how far you are from the origin; the second coordinate [tex]\theta[/tex] represents the angle between the positive direction of the x axis and the vector pointing to your point.
The point (-3,0) is clearly 3 units away from the origin, so we have [tex]\rho=3[/tex]
Finally, the point is on the negative side of the x axis, so you have to make an angle of 180° to reach it. Since we express angles in radians, the coordinates are
[tex](x,y) = (-3,0)\mapsto (\rho,\theta) = (3,\pi)[/tex]
