I tried much but was unable to find the answer. $$f(x) = \frac{1}{3} + \frac{1 \cdot 3}{3\cdot 6} + \frac{1\cdot 3\cdot 5}{3\cdot 6\cdot 9} + \frac{1\cdot 3\cdot 5\cdot 7}{3\cdot 6\cdot 9\cdot 12} \ldots \infty$$
We have to find value of $X$. I think its an expansion of something, but don't know which.
Thank you.
By the way,the Answer is $2$.
Hint. One may recall the following result, coming from the generalized binomial theorem: $$ \sum_{n=0}^\infty\binom{2n}n x^n=\frac1{\sqrt{1-4x}},\quad |x|<1/4. $$ then take into account @Henning Makholm's comment.