In a mass-spring system with friction and external forcing, the position x of the mass is governed by the differential equation x'' + 2x' + 10x = 2 cos (2t) + 4 sin (2t).
Determine the steady periodic solution of this equation, and express it in the form x(t) = C cos (ωt - α) where C and ω are positive and α is in the range [0, 2π).