istake in the question. The unknown variable mentioned in the question is x, not a. Therefore, the value of the unknown variable x is 3.
The answer is: 3
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Find all values of $a$ so that the lines specified by
\[\begin{pmatrix} 1 \\ 2 \\ a \end{pmatrix} + t \begin{pmatrix} 2 \\ X \\ 4 \end{pmatrix}\]and
\[\begin{pmatrix} 4 \\ 1 \\ 0 \end{pmatrix} + u \begin{pmatrix} 5 \\ 2 \\ 1 \end{pmatrix}\]are skew. The answer is 3. What is the value of unknown variable X?

### Answer:
