Bài giải

                                                                                               Độ dài đoạn MN là :

                                                                                                    2x2=4(cm)

                                                                                            Độ dài đoạn thẳng AB là :

                                                                                                   14x2:4=7(cm)

                                                                                               Nửa chu vi ABCD là :

                                                                                                     20:2=10(cm)

                                                                           Độ dài chiều rộng AD của hình chữ nhật ABCD là :

                                                                                                       10-7=3(cm)

                                                                                      Diện tích hình chữ nhật ABCD là :

                                                                                                    7x3=21(〖cm〗^2)

                                                                                                          Đ/S : 21 〖cm〗^2