For part 9(b), the question says "hence …".

When question says "hence...", we MUST use the result from the preceding part.

When question says "hence or otherwise ...", then we need not use the result from the preceding part.

Even though the answers for 9(b) are correct here, likely no marks will be awarded because did not follow the question instruction.

Correct method should be …

from 9(a),
x^2 -6x+2=(x-3)^2-7
so,
x^2-6x+2=0 implies
(x-3)^2-7=0
(x-3)^2=7
x-3=+sqrt(7) or -sqrt(7)
x=+sqrt(7)+3 or -sqrt(7)+3
x=5.65 or 0.35 (to 2 d.p.)