It is NOT slang. It is a dialect, Common Czech https://en.wikipedia.org/wiki/Czech_language#Common_Czech, my native dialect.
Common Czech uses the used-to-be-dual endings for adjectives and for numbers čtyři a pět.
From the Wikipedia page:
"• unified instrumental ending -ma in plural: s těma dobrejma lidma, ženama, chlapama, městama (with the good people, women, guys, towns) – stand.: s těmi dobrými lidmi, ženami, chlapy, městy. In essence, this form resembles the form of the dual, which was once a productive form, but now is almost extinct and retained in a lexically specific set of words. In Common Czech the ending became productive again around the 17th century, but used as a substitute for a regular plural form"
Well it was just missing. Also, from a few forms I tried this one is the least used https://books.google.com/ngrams/graph?content=came+by+car%2Ccame+in+car%2Ccame+in+a+car%2Ccame+with+car%2Ccame+with+a+car&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Ccame%20by%20car%3B%2Cc0%3B.t1%3B%2Ccame%20in%20car%3B%2Cc0%3B.t1%3B%2Ccame%20in%20a%20car%3B%2Cc0%3B.t1%3B%2Ccame%20with%20a%20car%3B%2Cc0 But I will add it.