a linear equation

difficult / hard, probably impossible

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

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

a quadratic equation

the first / (mathematics, attributive) the quality of being first-degree

(mathematics, chemistry) an equation

next, order, sequence, below

one day

next, secondary / following