Suppose I am in debt to someone to the value of $5. Then we can say that my net worth is -5. Now my friend wants to help me out with my debt by 'taking away' the -5. He can do this by giving me $5 and now my debt is zero.
so we can say that these 2 are equivalent -5-(-5) = -5 + 5 = 0.
Hope this helps.
