In the treble clef line , what key is the hash tag note?
I know it’s a black key but can’t remember which .
This hashtag is what we call a "sharp." It raises the pitch by one half step.
I'm assuming you mean the sharp in the bass clef. (Based on the music, it's hard for me to imagine these both being in treble clef.)
Since the written note is F, you'll move it up one half step to the nearest black key to make it F♯. If you're still having trouble finding this key, look for the group of three black keys; it will be the one on the far left.
If the left hand is in fact in treble clef, this pitch will be a D♯, which is the right-most black key in the grouping of two black keys.