How do I solve this?
The same way you solve your previous problem! You have a right triangle where one leg has length 34 and the other has (signed) length -21. Use the Pythagorean theorem to determine the length of the hypotenuse and use inverse trig functions to determine the angle.
Without knowing how the textbook defines things, it is hard to answer.
But if $(a,b)$ is a point then the angle determined by the position vector $<a,b>$ can be found as follows.
If $a\cdot b\ne 0$ then
$\theta=$ $\begin{cases}\arctan\left(\dfrac{b}{a}\right) &\text{ if }a>0 \\\pi+\arctan\left(\dfrac{b}{a}\right) &\text{ if }b>0~\&~a<0 \\-\pi+\arctan\left(\dfrac{b}{a}\right) &\text{ if }a<0~\&~b<0 \end{cases}$
So what is your answer now?