Answer: The number of hydrogen atoms must be 6.
Step-by-step explanation:
Alkenes are unsaturated hydrocarbons in which a single bond is present between carbon and hydrogen atoms and double bond is present between carbon and carbon atoms.
The general formula for writing an alkene is given as
where, n = 1, 2, 3, 4....
It is given that number of carbon atoms are 3, which means that n = 3.
Putting (n = 3) in above expression, we get:

Hence, the number of hydrogen atoms must be 6.