Question 637608
I think you have to find the distance between the two points ( A and A').
There is a formula in coordinate geometry called 'Distance Formula' which is used to find the distance between two points whose coordinates are given.
If A(x1,y1) and A'(x2,y2) are two points then
AA' = {{{sqrt((x2-x1)^2+(y2-y1)^2)}}}

Now A(9,12) & A'(3,6)
So
AA' = {{{sqrt((3-9)^2+(6-12)^2)}}}
= {{{sqrt((-6)^2+(-6)^2)}}}
= {{{sqrt(36+36)}}}
= {{{sqrt(72)}}}
= {{{6*sqrt(2)}}}