Verified answer

Use the dot product:

a * b = ||a|| ||b|| cos t.

a * b = 3 + 3 + 3 = 9

||a|| = sqrt(9 + 1 + 3) = sqrt(13)

||b|| = sqrt(1 + 9 + 3) = sqrt(13).

Thus, 9 = 13 cos t

==> t = arccos(9/13) = 0.806 radians.

I hope this helps!