Who ever said the solution had to be unique? There's an entire category of brainteasers intended to elicit as many alternative answers as possible.
And even if the answer must be unique, who says it's a number and not an analytic expression? "The area of the triangle" could be a number, an equation, or the text from a thread on an internet forum.
What's also interesting is the presumption that the numbers are base 10. What if the numbers are base 12 which makes the triangle "valid" in our normal 2-D Euclidean space? The answer in base-10 math would be 6*12/2 = 36 which converted back to base 12 is "30". And what if the numbers are base 16? The answer in base-10 math would be 6*16/2 = 48 which in base 16 is "30". For any base of 12 or greater, the triangle exists and the answer is "30".
P.S. The hexagonal tiles are clearly projections of cubes which implies the triangles are projections on some 3-D space.