Question 4. A mass m on a spring with spring constant k satisfies the differential equation mu" ku= 0, where u(t) is the displacement at time t of the mass from its equilibrium position. (a) Let x1 = u and x2 = u' and show that the resulting first-order system is 0 1 k x' = 0 m ✗. (b) Solve the system and draw a phase portrait. (c) For one non-trivial trajectory (not the origin), draw the corresponding component functions as functions of time. (d) Determine the relationship between the eigenvalues of the coefficient matrix and the frequency of the spring-mass system.