Questions & Answers about Подача была короткой.
Because короткой is in the instrumental case, which is very commonly used for the predicate adjective after быть in the past tense.
So in:
Подача была короткой.
- подача = subject, nominative
- была = was
- короткой = adjective agreeing with подача, but in the instrumental
This is a very standard pattern in Russian:
- Игра была интересной. = The game was interesting.
- Комната была маленькой. = The room was small.
A nominative form like короткая is sometimes possible in Russian in certain contexts, but короткой is the normal form learners should expect here.