I do this one a little differently from Melody.......but the answers are the same.
First...note that S30°W = 240° .....and S20°E = 290°
So we have two vectors here with the following components....
<300cos240°, 300sin240° > and < 50cos290°, 50sin290° >
And adding the first to components of the two vectors we get.....300cos240° + 50cos240° = -132.8989928337
And adding the second components of the two vectors we get.....300cos290° + 50cos290° = -306.7922521745
And to find the magnitude of the resultant vector, we have.......
√[( -132.8989928337)2 + ( -306.7922521745)2 ] = √(111783.6282905138439343) ≈ 334.34 km/h
And to find the resultant heading, we have
tan-1 [(-306.7922521745) / ( -132.8989928337)] = tan-1 (2.308461829792775) = 66.578275555105°
And to find the resulting heading, we have.... S(90 - 66.578275555105)°W ≈ S(23.42)° W
I actually like this method better because I always have a little trouble visualizing the "parallelogram law." In effect, I just let the math "do the work" for me and I don't have to worry about drawing any pictures.
But, it's purely a matter of preference....!!!!