First of all, you would need arctan, not tan. Second, you can't just divide these two, this only works in the first quadrant.You're in the third quadrant. Arctan on wolfram alpha and Mathematica (wolfram language) can also accept x and y separately:
https://www.wolframalpha.com/input/?i=arctan(-3.5,-2.5)+in+degrees
which will give -144.5 degrees, or equivalently -144.5+360 = 216 degrees
Read the last line of your book carefully to understand why x/y does not work in this quadrant.
