Let x₁, x₂ be the roots of $${x^2} - 3x + a = 0$$ and x₃, x₄ be the roots of $${x^2} - 12x + b = 0$$. If $${x_1} < {x_2} < {x_3} < {x_4}$$ and $${x_1},{x_2},{x_3},{x_4}$$ are in GP, then ab equals