Answer:
Explanation:
First equation:
 15 - x = - 3
 -x = - 3 - 15 
 -x = -18 (now negative signs on b.s cut eachother)
 [x = 18]
Second equation :
 15 - x = 3
 -x = 3 - 15 
 - x = -12 ( now negative sign on b.s cut eachother)
 [x = 12]
so x = 18 and x = 12 are not same