Pay attention to look at both sides of the formula. One side is addition, and the other side is subtraction. 
The house number of a community consists of $5$ digits: $1, 2, 3, 6, 7$. Which number is not used if these $5$ digits are filled in the squares below to make the equation correct? Each number can only be filled in once.  $$\huge\square -\square =\square +\square $$ 
a: $$7$$ 
b: $$6$$ 
c: $$2$$ 
d: $$3$$ 
e: $$1$$ 