Answer:
Addition property of equality:
if a =b then; a + c= b + c
Subtraction property of equality:
if a =b then; a - c= b - c
Substitution property of equality:
If x =y then, x substituted in for y in any equation and y can be substituted for x in any equation.
Multiplication property of equality:
if a =b then; 

Division property of equality:
if a =b then; 

Commutative property of addition
 a + b = b+ a
Given the system of equation: 
 ......[1]
 ......[1]
 .....[2]
 .....[2]
According to Multiplication property of equality;
Multiply equation [2] by -2 to both sides we get;
 ......[3]
 ......[3]
Using Commutative property of addition;
Add equation [1] and [3] we have;
 
 

By division property of equality:
Divide by 13 both sides we get;

using Substitution property of equality:
Substitute the value of y in equation [1], we get;


By Addition property of equality:
Add 3 to both sides we get;

Simplify:
x = 7