Answer: 8x4
Explanation:
Draw a rectangle. Label the smaller side length 4, and split the other side into two parts
One section of this split rectangle represents the 3x4, piece, the other is the 5x4 piece. Label your split side lengths 5 and 3. These two smaller rectangles within the big one are the smaller arrays
To find the total size of the original, you find the total length. We know one side is 4, and the pieces fit together, so the other factor must be the combined length of the two smaller rectangles
Add the 3 and 5. 3+5=8
The original was therefore 8x4 :) Sorry I cant draw it out so I hope my description made sense