'Demonstrable' -a form of 'demonstrate'?

Hello,
Please look at this:
Put the correct form of the suggested words to the blank:
What you say is …false.Let me show you the facts. (demonstrate)
I think the answer is ‘‘demonstrably’’ because it sounds good in that context,but I wonder whether it is a form of ''demonstrate" ?

.
Yes, it is:
dictionary.cambridge.org/define. … &dict=CALD
.