The differences between consecutive terms are: $3-1=2$, $6-3=3$, $11-6=5$, $18-11=7$, ?, $42-X$. The differences 2, 3, 5, 7 are successive primes, so the next difference is 11: $X = 18+11 = 29$. Check: next difference 13 gives $29+13 = 42$, consistent. Hence X = 29.