A student solved the equation x + 2 (x + 1) = 17 below and showed their work in the table below.
Problem x + 2 (x + 1) = 17
Line 1: x + 2x +2 = 17
Line 2: 3x + 2 = 17
Line 3: 3x = 15
Line 4: x = 5
Identify the property used to arrive at line 2.