Platelet area using ImageJ program

Good day everyone!
I have a question about ImageJ. How to calculate the total area of the particles (platelets) in the figure and the area of the remaining surface (clean without platelets)?

Depending on what the ground truth actually is, you might some sort of pixel classifier like Weka or Ilastik (many posts on pixel classifiers, check them out). Kind of a tangent, but if everything there is either a platelet or background, it might be easier to find the area of the background and the entire image, and then use those two values to find the platelet area, rather than trying to classify the platelets directly. The background looks most consistent.