The puzzle was not difficult enough to _______ the experienced solver.

• Enough (যথেষ্ট) এর ব্যবহার: 
- Adverb/adjective এর পর 'enough' বসলে to-infinitive (to + verb এর base form) ব্যবহৃত হয়। 
Structure - adverb/adjective + enough + (to+verb+....) 
Example - He didn't run fast enough to win the race.

আবার, adjective/adverb/verb + enough + for + pronoun (me/him/her, etc) + (to+verb) এরূপেও ব্যবহৃত হয়। 
Example - What he says is not clear enough for me to hear him clearly.

• প্রদত্ত sentence এ 'difficult' (adjective) এর পর 'enough' শব্দটি এসেছে। তাই to-infinitive (to + verb এর base form) ব্যবহৃত হয়।

Complete Sentence - The puzzle was not difficult enough to challenge the experienced solver.

Source: A Passage to the English Language by S.M. Zakir Hussain.