Triangle ABC
Apply rule of Cosin
If 12^ + 9^2 - 2•12•9 Cos X= 18^2
Then
144+ 81 - 216CosX = 324
CosX = (324 -225)/(-216) = 99/-216
CosX = -0.46
Then answer is YES, its a triangle because CosX only have values
between -1 and 1, and -0.46 is between them