In order to solve this equation you have to clear X and to clear X you have to add 28 in both sides of the equation to eliminate the -28 from the side of the x:
375=x-28
375(+28)=x-28+(28)
375+28=x
x=403
The student instead of adding the 28 put them with negative sign making him withdraw from teh 375 instead of adding, that was his mistake.
