If f is any face, then the degree of f (denoted by deg f) is the number of edges encountered in a walk around the boundary of the face f
No. edges = no. of region*(deg/2) (since each edge is part of exactly 2 regions in a planar graph and hence contributes 2 to the sum of degrees)
We have Euler's formula for planar graph
$n - e + f = 2$, where $n$ is the no. of vertices, $e$ is the no. of edges and $f$ is the no. of faces (regions in planar representation)
And we have $2e = 6f = 6 \times 35 = 210 \\ \implies e = 105$.
Now, $n = 2 + 105 - 35 = 72$.