PEMDAS is the correct order of operations... parentheses, exponents, multiplication, division, addition, and subtraction.
Let's do Holly first
6 x (24/2)^2= first the parentheses
6 x (12)^2 = now exponents
6 x 144 = and multiplication
864
Now Jim
6 x 24 / 2^2 = exponents first
6 x 24 / 4 = next multiplication
144 / 4 = now division
36
So, Jim's expression is 828 less than the other expression
(864 - 36 = 828)