Police are seeking information about two attempted robberies near Lathlain last month.
A man threatened a 17-year-old with a knife at Kustom Cupcakes on Orrong Road just after 11am on August 21.
He demanded money from the till but fled empty-handed after the girl yelled out to her colleagues.
About five minutes later, the man approached a 70-year-old woman as she got into her car on Gladstone Road in Rivervale.
He asked the woman to drive him to the shops and tried to get her to hand over cash while clutching a knife.
The man ran off when the woman screamed for help.
He is described as aged between 40 and 50 with fair skin, a slim build, blonde or grey facial hair and rotten and jagged bottom teeth.
It is believed the man is also missing some of his top teeth.
He wore a grey hooded jumper, dark blue tracksuit pants and black sunglasses.
Anyone with information should contact Crime Stoppers on 1800 333 000.