Observe the differences between consecutive terms:

$3 - 1 = 2$ $6 - 3 = 3$ $11 - 6 = 5$ $18 - 11 = 7$ $X - 18 = ?$ $42 - X = ?$

These differences are consecutive prime numbers: $2, 3, 5, 7, 11, 13, ...$

So: $18 + 11 = 29 \rightarrow$ This gives the value of X. Check next: $29 + 13 = 42 \rightarrow$ Matches the sequence.

Therefore, $X = 29$.