What is the missing value

72

Explanation:

The pattern here may be hard to find at first, but it's this: the number in the middle of the triangle = (number at lower left corner of triangle x number at upper vertex of triangle) + (number at upper vertex of triangle x number at lower right corner of triangle).

Thus, for the missing value...

Missing value = (3x8) + (8x6) = 24+48 = 72.

Could you tell me what concept in chemistry relates to this? I'm interested.

Also check out stylesben's answer. Seems like there's several ways of doing this.

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.