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