For vectors u=-6i + 2k and v=7i-4j, find (-3u) * v.

A) - 450

B) - 50

C) 108

D) 150

I know how to do these problems, but I am stuck on this question, because I keep finding the answer 118. I know you change - 6i+2j to (-6,2) and 7i-4j to (7,-4) and then plug them into the equation but I don't know what I am doing wrong.

You can compute the dot product in the usual way, then multiply the result by - 3, or you can multiply u by - 3, then compute the dot product. Either way, it is 150.

-3 (u•v) = - 3 * ((-6) * 7 + 2 * (-4)) = - 3 * (-42 - 8) = - 3 * (-50) = 150

(-3u) •v = (18i - 6j) • (7i - 4j) = 18*7 + (-6) (-4) = 126 + 24 = 150

Selection D is appropriate.