You don't need quotes around 48.
Range("C8").Formula = "=TRIM(MID([@AR],SEARCH(48,[@AR])-2,12))"
By the way, if you are searching for a real text value, you need to double the quotes inside a quoted string, for example
Range("C8").Formula = "=TRIM(MID([@AR],SEARCH(""an"",[@AR])-2,12))"