They were fighting Hatchiyack after the others had given them their energy (look up LR TEQ Trunks and Broly to get a better look on the characters from the back) when they'd finally beat him with a powerful ki blast from both of them

Trunks and Broly: Revert back to base form

Trunks: We... we did it!

Broly: Looks over towards Trunks I'm glad it's finally over...

Trunks: Notices you Who are you?