i'm stuck on this proof:

prove that any planar graph with very vertex having degree 5 has at least 12 vertices.

a planar graph G with all v vertices with degree 5 has 5v/2 edges

Euler's formula states that vertices - edges + faces = 2

so there are 3v/2 + 2 faces in G

I'm not sure how to get the # of faces in G, or whether it's relevant to the problem.

also this one:

show that a plane graph with v>=3 vertices has at most 2v - 4 edges

any help is appreciated.