My new dog is part greyhound,part bull mastiff how do I make people believe I am not starving her?

This is a problem you'll have for as long as you have your dog (congratulations on the new dog though- what a cool mix!) You have to remember that more than 50% of the general public who has dogs is clueless about breeds other than their own and an additional 50% of the general public doesn't even have dogs and knows nothing about them. Those numbers don't bode well for the ratio of people who meet your dog to the people who know about greyhounds. You'll probably have to get used to people not having a clue.