Total ways of selecting 3 vertices: 16C3 = 560
but it included the linear vertices. (3 vertices in same line)
total 10 lines (4 vertical + 4 horizontal + 2 digonal ) which contains 4 elements
so remove these = 10 lines × one line contains (4C3) = 40
and there are 4 cross lines (up and down to digonal) which contain exact 3 vertices = 4×3C3=4
So Net total = 560-40-4 = 516