You have photos, right? I'll tell you directly according to the picture.

First of all, the Q point is any point on the OQ in the topic, but we can "fix" it first:

The parallel line that passes through point P and makes the Y axis intersect OQ is at point Q, and the vertical X axis is at point A, so the abscissa of point Q and point A is -3 like P.

So the ordinate of point Q is -3Xtanβ=-3X-2=6.

Find the Q coordinate (-3,6) of this point.

Yi PQ= 10, OP=5, OQ=3√5.

∴PQ？ =OP？ +OQ？ -2OPXOQXcos∠POQ

The solution is cos∠POQ=-√5/5.