What confuses me is the extra line in the pentagon (double bond???) How does this extra line affect how you would write the molecular formula?
Yes, the extra line means a C=C bond(double bond). To put it simple, since carbon must have 4 bonds, if it has more bonds with carbon or other atoms, it has less bonds with hydrogen.
Consider the following diagrams. When a C-C bond(single bond) is replaced with a C=C bond, each of the two carbon atoms have 1 less hydrogen atom bonded to it, since carbon must have 4 bonds only. Try to count the number of 'lines'(bonds) besides each carbon atom.
Going by what you say if no more hydrogens can attach the formula would be c3h8? Again im not sure of the impact of the double line in the pentagon, as to whether this affects the formula.
C
3H
8? Are you referring to the propene structure you drawn or to the structure of the original question?