understanding, comprehension / appreciation

(mathematics) lemma

solve (a problem); answer (a question); unravel (a riddle) / comb; untangle (hair)

untie; unravel; undo (a fastening); unwrap / dispel (doubts); clear up (a matter)

understand, comprehend / appreciate

first of all; anyway; about

needed, necessary

need, necessity

mathematics (field of study)

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