How to cut pixels from an image

Hello, so I have no idea how to do this. In the image, there's a red box. that would be my center point. i have to just extract the one in blue box. so the part in the green box should be deleted. like, the one in blue box should have the "same size" as the one in the red box. can anyone help me? i tried using threshold, but what should i put there? 0?

please help me :(