What do it mean by "If equals are subtracted from equals, then the remainders are equal . "?

Answer :

According to fourth Axiom from the Euclid' axiom we know " If equals are subtracted from equals, then the remainders are equal . "

That states that if we have x  = y   And  a  = b ,
So according to axiom we get

x  - a  = y  - b  

Let we have x + 2 = 9   And   2 =  2 
So,

x + 2 - 2  = 9 - 2 

x = 7  ( we can see that both are same after subtraction equal values )