The maximum value of the objective function is _____

Question

Consider an objective function Z(x₁, x₂) = 3x¹ + 9x₂ and the constraints

                       x₁ + x₂ $\leq$ 8,

                       x₁ + 2x₂ $\leq$ 4,

                       x₁ $\geq$ x₂ $\geq$ 0,

            The maximum value of the objective function is _____