Let the 3 sides of a triangle be a,b and c 
For it to be a triangle then 
a + b > c , a + c > b , b + c > a 
 12 , 12 , 20 
12 + 12 > 20 
24 > 20 
12 + 20 > 12
32 > 12 
Hence is a triangle 
10,3,7 
3 + 7 .....10 
10 = 10
Hence is not a triangle since it does not satisfy the rules above
7,7,11
7 + 7 .... 11 
14 > 11 
11 + 7 ..... 7
18 > 7 
Hence the figure formed is a triangle 
24,9,12 
9 + 12 .... 24 
21 < 24 
Hence is not a triangle 
The rule is that for 3 numbers to form the sides of a triangle then the sum of any 2 of the numbers should be equal to the third