My solution
↓
Given (−2.67b + 11) − (3.71b − 15)
Step 1 • -2.67b + 11 - 3.71b + 15 Margie's error is that she did not multiply -15 by -1 to get +15. She instead got -15
Step 2 • -2.67b - 3.71b + 11 + 15 No error except the +15
Step 3 • (-2.67b - 3.71b) + (11 + 15) I don't understand what this step is for: Margie has already grouped the like terms in the last step. No error except the +15
Step 4 • -6.38b + 26 No error except the +26 that came from the +15 mistake
Step 5 • 26 - 6.38b Shuffling the terms around to make it a bit neater
Margie's error was in step 1. Her error is that she did not multiply -15 by -1 to get +15. She instead got -15