How does he not expect help..? You could help him understand the question of course...
Okay so I'm assuming you know how to do cross products.
Let and and and
Then it says the sum of the two vectors is <-4,3,0>. so
Then it says the first vector is parallel to <-3,-1,5>, so we use the dot product:
Now the cross product. The theory is that
Doing that will give you three more equations. Solve for for and for . Finally, plug those values in the first 3 equations. Then, solve for , , and and plug into the equation we got from the dot product.