how do I solve this proof?:

If a+b=c, then a=c-b

By shifting b from one side of the equation to the other side, it has to change the sign from+ to -.

If a + b = c then c - b = a

a + b = c

a + b( - b) = c( - b)
Postulate: What you do to one side of an equation you must to to the other.

a + 0 = c - b

a = c - b