A=4
B=0
C=7
X=16
This is what I think
Answer:
a = 
Step-by-step explanation:
Using Pythagoras' identity in the right triangle.
The square on the hypotenuse is equal to the sum of the squares on the other 2 sides, that is
a² + 9² = 12²
a² + 81 = 144 ( subtract 81 from both sides )
a² = 63 ( take square root of both sides )
a = 
The answer would be the 3rd number line or "C"
The 2 smallest lengths in a triangle have to add up to be greater that the longest side of the triangle. In this case, the longest side is 20. The 2 smallest sides are 5 and 10. 5+10=15, but 15<20. This is why these 3 lengths can’t create a triangle.