Derive a formula for calculation of new co-ordinates of a point when the c-ordinate axes $x$ and $y$ are rotated by an angle $\theta$.

Question

I was studying rotation of co-ordinate axes. There was an article given in https://en.m.wikipedia.org/wiki/Rotation_of_axes about it. However, I don't get the derivation section, the picture of which is attached below:

However, I do not get how they write that $x',y'$, system, $P$ will have coordinates $(r,\alpha-\theta)$? Wouldn't it be $(r,\alpha+\theta)$? I am not quite getting it...