Hello, I have a polygon

shape file with the basin boundaries of a country. the attribute table contains basin names and areas. at first I need to select places with an area less than 10000km2. I did this with select attribute and created a new polygon. then I need to select the basins with an area less than 10000km2 and the locations of the basins with each other are less than 50km. but I don't understand how to do this with select location. if you can help me, I would be very grateful.