Line 1: x+2x+1=17 + 2 + 1 = 17

Question

Line 2: 3x+1=17 3 + 1 = 17
Line 3: 3x=16 3 = 16
Line 4: x=163

Answer 1

Your calculations in lines 2 and 3 are incorrect.

Correct calculations:
Line 1: x + 2x + 1 = 17
Line 2: 3x + 1 = 17
Line 3: 3x = 16
Line 4: x = 16/3 or x = 5.33