Question

If x+3=y then x=y-3. what definition, property, postulate, or theorem justifies this

Answer 1

Notice that to go from the math statement:

x + 3 = y

to the statement

x = y - 3

what we have done is to subtract from both sides the value "3"

So what we have applied is called: "The subtraction property of equality"

which tells us that as long as we subtract the same value from both sides of the equal sign, the equality stays true.