To find 19 + (11+37), lennie added 19 and 11. then he added 37 to the sum. what property did he use?

Lennie used associative law of addition.

Associative law of addition doesn’t matter how you group the given numbers to add.

a + (b + c) = (a + b) + c

=> 19 + (11 + 37) or (19 + 11) + 37

=> 19 + (48) or (30) + 37

=> 67 or 67