What will come in place of the x in the following Number…

What will come in place of the x in the following Number series? 12, 38, 116, 350, 1052,3158, x

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #11872