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.