Answer:
19
Step-by-step explanation:
let the smallest square be A, medium square is B and the largest is C ;
as the length side of the main square is 28, we can find that,
A= 28-22 = 6
C= 28-15 = 13
So to find B,
A+B+C = 28
B = 28-A-C
B = 28-6-13 = 9
So the line with questioned mark is,
28-9 = 19
