52, 25, 77, 77,?, 451, 605

The missing number is 154.

Step-by-step explanation:

Adding the 52 and 25 gives 77.

Then reverse the 77 to get the next term (77).

Now we add the 2 77's to give 154.

So the next 2 terms after the second 77 are 154 and the reverse 451.

The missing number is 154.