go at vs knuckle down to

go at

verb
  • Used other than figuratively or idiomatically: see go, at. 

  • To try to solve a problem a specific way; to undertake a task. 

  • To initiate an attack on or an argument with. 

knuckle down to

How often have the words go at and knuckle down to occurred in a corpus of books? (source: Google Ngram Viewer )