Keep at
Definition 1 of 2
Pro Tip 1/3Steady Persistence
Used to express continuous effort despite difficulties. 
Even when the math problems were tough, she kept at it.
Pro Tip 2/3Encouragement Phrase
Often used to motivate someone to maintain their focus or effort. 
You're almost there, just keep at it!
Pro Tip 3/3Determination
Highlights a person's determination when facing challenges. 
He kept at his invention, despite the failed tests.