Explanation:
 length 
a + b = 18cm 
 a = 18 - b 
 Area 
a² + b² = 212cm² 
 (18-b)² + b² = 212 
 324 - 36b + b² + b² = 212 
 2b² - 36b + 112 = 0 
 b² - 18b + 56 = 0 
 b² - 4b - 14b + 56 = 0 
 b(b-4) -14(b - 4) = 0 
 (b-4) (b-14) = 0 
 b-4 = 0 OR. b-14 = 0 
 b = 4 cm b = 14 cm 
so 4cm < 14cm 
 so , length of a side of the smaller square is 4cm ....... 
 And , large square's = 14cm .....