yes, that's the answer. i wish i had my graphing software and stuff on this PC so that i could show you the diagram.

anyway, plot the center, and draw a line connecting it to (-3,0). the chord is perpendicular to this line. so draw that (the line you drew first is the perpendicular bisector of the chord). then use the formula , with and as shown here. you can figure out the value of d using Pythagoras' theorem, and then just solve the equation for r