For question 20 a), The answer in the back of the book is 16 degrees, I am getting 22 degrees. My approach to the solution was to calculate the point of intersection which I got (4, -3/2 , 1). Then I let y=z=0 on the equation of the plane to find a point on it which in this case is (2,0,0). So, we have two points on the plane (2,0,0) and (4, -3/2 , 1) and so we can find another direction vector of which I calculated (2,-3/2 ,1).

Now using the formula $\displaystyle cos\theta = \frac{d1 (dot) d2 }{|d1| \times |d2|}$ I get 22 degrees.

Why am I wrong or why is my approach incorrect.

Thank you