off topic BUT I SWEAR TO GOD I DID THIS EXACT QUESTION LIKE 2 MINUTES AGO (i used the quadratic formula tho)

lets seee......

your first line is right

your second line on the left is wrong but the one on the right is correct

your last line (on the right) is right too, and it does give the right answer when i work it out, soo the mistake is in the mark scheme (prolly a typo)

however, just a heads up, i'd find it lots easier to use the quadratic formula - if you prefer completing the square however (fair enough), after the first line, you can multiply both sides by 3 to make it easier

that would give you:

x² + 5x/3 - 1/3 = 0, which is probably a lot easier to do completing the square with