A number a + bi in rectangular form converts to the polar form

(√(a^2 + b^2))@ArcTan[b/a]

Your complex number converts to

(√((-3√3)^2 + 3^2)) @ ArcTan[3/(-3√3)]

= √(27+9) @ ArcTan[1/-√3]

= 6 @ 150°

I chose to use this "@" notation for expressing the answer in the form (magnitude)@(angle). The notation preferred by your text or instructor may be different.

(√(a^2 + b^2))@ArcTan[b/a]

Your complex number converts to

(√((-3√3)^2 + 3^2)) @ ArcTan[3/(-3√3)]

= √(27+9) @ ArcTan[1/-√3]

= 6 @ 150°

I chose to use this "@" notation for expressing the answer in the form (magnitude)@(angle). The notation preferred by your text or instructor may be different.