Let p = (2, k) and q = (3, 5). Find k such that p and q are parallel.

Should be easy to solve, and I have all the formulas here, but alas, no luck.

A second exercise is to find k such that the angle between p and q is pi / 3, but I suspect I will be able to solve that one once some kind person helps me figure out what I am doing wrong in the first one.

Doh! So simple.

y = 5x / 3, which gives k = (5*3) / 2 = 10 / 3.

I was caught up in thoughts using a zero angle between the vectors, since that was what the chapter in book was about. Anyone care to help show how to solve this problem using the dot product?