Answer:
2
Step-by-step explanation:
the midpoint formula :
(x1 +x2) /2 , (y1 + y2)/2
finding the midpoint of AC.
a's coordinates are : ( 0,0) -- where 0 (left) is the x axis , 0 (right) is the y axis
c's coordinates are : ( a,a)-- where a (left) is the x axis , a (right) is the y axis
( a - 0)/2 , (a - 0)/2
=a/2 , a/2
hence the value in the green box is 2
