Simplify the following text for a Grade 9 native English-speaking person.
Original: The era of the slide rule ended with the launch of pocket-sized scientific calculators , of which the 1972 Hewlett-Packard HP-35 was the first .
Simplified: The last nail in the coffin for the slide rule was the launch of pocket-sized scientific calculators , of which the 1972 Hewlett-Packard HP-35 was the first .