I just want to where do i start on green lantern when mongrel destroys Central city
Mongul destroys Coast City in Superman 80.
The corresponding Green Lantern issue may not exist in this library.
Green Lantern 46 is not available in our library.
Green Lantern 48, with a different writer, deals with Coast City destruction also
Via DCU, the earliest point you can jump into the Green Lantern series that directly deals with Coast City is Green Lantern (1990) #48.
As @Turok said, that issue is where a new writer began (Ron Marz, who would go on to be associated with Green Lantern for years and years) and where the aftermath of Coast City’s destruction began to have profound, lasting effects on Hal Jordan.
If you’re wanting to cut to the chase and read something from this same series with Mongul, there’s issues 52 and 53 (Superman guest stars in 53). However, the Green Lantern in these issues is the newly minted Kyle Rayner, as Hal was…“preoccupied” at that time and not on Earth, in his right mind or a GL for that matter.