Forces are vector quantities, so when many forces act on a body their combination is not always an arithmetic sum.

Combining parallel forces

                                          5N                                                          8N

7N                                                10N                                   resultant

                                                                                                         force

1. Resultant force= 10+5+(-7)= 8N

7N                                               10N                                  Body in

3N                                                                                            equilibrium

2. Resultant force= 10+(-3)+(-7)=0