Cant Get rid of Blackborders

Hi, im new to this app, and i tried everything i cant get rid of my black borders afther im done editing, i have black borders on right and left and i dont know much about this but i tryed aspect ratio of 16:9 and 4:3 and still wont go away, can anyone guide me throught step by step to solve this problem please?

What are you doing to get the black borders in the first place?

However, for example: After using the Crop filter tool, you can use the Size and Position filter.