4 Section 3.12, Systems of differential equations. The nonhomogeneous equation. variation of parameters. Page 366

  4.1 problem Example 1, page 361
  4.2 problem Example 2, page 364
  4.3 problem 1
  4.4 problem 2
  4.5 problem 3
  4.6 problem 4
  4.7 problem 5
  4.8 problem 6
  4.9 problem 10
  4.10 problem 11
  4.11 problem 12
  4.12 problem 13
  4.13 problem 14
  4.14 problem 16
  4.15 problem 17
  4.16 problem 18