It is not X2 - 7.5 = 0 or X2 = 215

Please answer the following question:

In the following picture for two dimension points on planes (X1, X2), the circle and triangle represent support vectors for a dataset of two classes. Given the equations for the hyperplanes X2 -12 = 0 and X2 - 3 = 0 for the circle and triangle, without much calculation find the missing value in the following equation of the decision boundary drawn in the middle.

The equation is X2 - ______?______ = 0