Cartesian coordinates

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

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

to use / to employ / to make use of, to apply / to spend

being orthogonal

to intersect orthogonally

(mathematics) coordinate

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

use one thing for many purposes / work someone hard

to use one's head