Problem Link — Domino-and-tromino-tiling-problem

In this edutorial there is two problem.

part-1) Some tiling problems (I understand this completely)

part-2) A more complicated tiling problem (I have doubt in this)

the recurrence relation is f(n) = f(n-1) + f(n-2) + 2g(n-2)

I have doubt in function g(n) → (covering n*2 grid using L-shaped tile) is the recurrence reletion (g(n-2) part explain little bit) is correct.

I got some explanation for this question but i have some doubt; please Help!!!

- The **Dp formula** for this question is this dp[n]=dp[n-1]+dp[n-2]+ 2*(dp[n-3]+…+d[0])

-  according to my understanding the dp[n-1] and dp[n-2] is due to domino

- and the 2*(dp[n-3]+…+d[0]) is due to tromino

- can someone explain why the total number of tiling for the trominos is 2*(dp[n-3]+…+d[0]);

or if my understanding about Dp formula is wrong please correct me!!

please help!!!