Find the missing number in the series: 2, 5,__ , 19 , 37, 75

This is a "series" where the value of a number (after the first two given numbers) is the result of an operation involving the preceding two numbers. 

If you have 
a and b 
then 
c = 2a + b 
Example :
Let's assume a=19, b=37.
So, c = 2*19 + 37 = 75 
which is the given next number. So, our theory is correct and now to calculate third number after 2 and 5.
2*2 + 5 = 9 
we now have 
2, 5, 9 
We can also check it by assuming a =9 and b= 19 and then,
c=2*9+19=18+19=37 which is the next term in the given series.
Hence, the missing number in the given series is found to be 9.