I just need help on #3.

To find the Φ of the equation wouldn't we do tan Φ = a2/a1? If so how come the answer in the back of the book says sqr2 sin(t +3pi/4).

When I did the tan Φ = a2/a1 = -1/1 I would get a Φ angle of -0.78539.

Please confirm if I did something wrong or the book is wrong. Thank you!