Note that, P(X = -0.5) is more complicated than what you presented.

As mentioned in my earlier comment to the OP,

P(X = -0.5)
= P(1 appears with total not equal to 3 or 4)
= P(1,4) + P(1,5) + P(1,6) +P(4,1) + P(5,1) + P(6,1) + P(1,1)
= 1/6 + (4 + 5 + 6 - 3)/36 + 3/6 × 1/6 + 1/6 × 1/6
= 3/18

and consequently,

P(X = 0)
= 1 - 1/36 - 1/12 - 1/18 - 3/18
= 2/3

And hence, there is no mistake in the answer key.

okie ty

ty