2. Let y₁(x) = e-*cos(3x) be a solution of the equation y(4) + a₁y (3³) + a₂y" + a3y + ay = 0. If r = 2-i is a root of the characteristic equation, a₁ + a2 + a3 + as = ? (a) -10 (b) 0 (c) 17