Vector c = (4/| ab |) ab = [4/(2 √ 3)] (2, 2, -2) = (4 √ 3/3, 4 √ 3/3, -4 √ 3/3).

Note: The vector formula between two points AB=(x? -x？ ，y？ -Really? ，z？ -z？ )

Vector a | a | = √ (the modulus formula of x? +y？ +z？ )