Final answer:
To correctly punctuate the sentence, a semicolon should be added after '2.'
Step-by-step explanation:
The sentence given is: Laura Dewey Bridgman lost her sight and hearing at the age of 2 ___ but learned to read and write at the Perkins School for the Blind in 1837.
To correctly punctuate the sentence, a semicolon should be added after '2.' The revised sentence would be:
Laura Dewey Bridgman lost her sight and hearing at the age of 2; but learned to read and write at the Perkins School for the Blind in 1837.