physically painful, agonizing / difficult, strenuous, mentally trying / mentally agonizing / tight (budget etc.), straitened / forced

no problem; no objection; it doesn't matter

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

(informal) no problem

unthinkable, out of the question

practical question (problem)

matter of life or death / (idiomatic) an extremely urgent matter

viewing as a problem

to view as a problem

set of problems