If a graph's every vertex has degree of at least $6$, then it cannot be planar graph because it needs to satisfy Euler's formula.
See this MO question's comment by Noam Elkies for details.
If a graph's every vertex has degree of at least $6$, then it cannot be planar graph because it needs to satisfy Euler's formula.
See this MO question's comment by Noam Elkies for details.