Deleting Selections on One Layer

[Sean Piecuch]

I have a 3-layer project, with images that need to be simply trimmed with a rectangle selection on each layer. (I'm making a triptych image of three action shots made with fast repeat drive exposures on my camera.) When I isolate on one layer, execute the selection, it will simply not delete. I'm about to throw away my keyboard. I've worn out the backspace and delete keys. OS: Sonoma 14.1.1. Please help. Everywhere I look, I'm told to simply delete, and it simply does not work.