Explanation:
Given the information:
- 1st square: 12 square units
- 2nd square: DOUBLE that of the first square = 2*12 = 24 square units
From that, we can determine the length of the side in each square:
1/

<=> x = 2
2/

<=> a = 2
Please have a look at the attached photo.
Hope it will find you well.