Margie's work for adding linear expressions is shown below. After checking her answer with the answer key, she solved it incorrectly.

Given (−3.12b + 12) − (4.36b − 11)
Step 1 −3.12b + 12 + (−4.36b) + 11
Step 2 −3.12b + −4.36b + 12 + 11
Step 3 (−3.12b + −4.36b) + (12 + 11)
Step 4 7.48b + 23


Identify and explain the first step where Margie made an error.