Answer:
10 inches is the side length of each square windowpane.
Step-by-step explanation:
Length of the single window plane = x
Width of the single window plane = x
On combining all the windows pane ,length becomes , l= 3x
On combining all the windows pane ,width becomes , b= x
Length of the frame = L = 6 + 3x+ 6 = 12 + 3x
Width of the frame = B = 6 + x+ 6 = 12 + x
Area of the frame, A = L × B = 
Given sum of combined area of the 3 windowpanes and frame = 








x = 10 (accept)
x = -26 (reject)
10 inches is the side length of each square windowpane.