how does x+(-15)=40 have the same answer as x-15=40

Question

 and what is the answer

 

Answer 1

Subtracting a number is the same as adding the negative of that number.

Solving for x:          x+(-15)=40          original equation

                             x-15=40                re-write in simplest form

                             x-15+15=40+15    add 15 to both sides to isolate the x term

                              x=55                    solve for x