3. Разложите на множители: a) $$2xy - 6y^2$$; б) $$a^3 - 4a$$.

a) $$2xy - 6y^2 = 2y(x - 3y) = \textbf{2y(x - 3y)}$$ б) $$a^3 - 4a = a(a^2 - 4) = a(a - 2)(a + 2) = \textbf{a(a - 2)(a + 2)}$$