DISCUSSION
Identify the directions for the er and eθ : unit vectors: er points outward from O to P and eθ is perpendicular to er and pointing in the positive θ direction.
Note that the path of P is defined in terms of the radial distance r as being a function of θ. Therefore, in order to determine the time derivatives of r, you will need to use the chain rule of differentiation.
Please ask and answer questions here. Either way, you learn.