If the vectors u and v determine an angle of 60° and |u|=5, u • v = 15, then the value of 2v • (3u - v) equals:
u•v = |u| |v| cos60°, so
5|v|/2 = 15
|v| = 6
Now you have
2v•(3u-v) = 2v•3u - 2v•v = 6v•u - 2v•v = 6*15 - 2*36 = 18
5|v|/2 = 15
|v| = 6
Now you have
2v•(3u-v) = 2v•3u - 2v•v = 6v•u - 2v•v = 6*15 - 2*36 = 18