3.3 Phase Portraits for Linear Systems with Real Eigenvalues 295 20. The slope field for the system y dx =2x+6y dt dy =2x-2y dt is shown to the right. (a) Determine the type of the equilibrium point at the origin. (b) Calculate all straight-line solutions. (c) Plot the x(t)- and y(t)-graphs (t ≥ 0) for the initial conditions A = (1,-1), B = (3, 1), C=(0, -1), and D = (-1,2). x 21. For the harmonic oscillator with mass m = : 1, spring constant k = 6, and damping coefficient b = 7, (a) write the second-order equation and the corresponding system.