That's what I meant. In your notation of (a+3n) it corresponds to a ≡ 0 (mod 3). If you solve it by hand, it may take considerable time. Try to find the stretching value for, say, 233. Why not to let machines to do such boring tasks for us? It's what they are for, aren't they?
BTW, talking about machines, I tried to run Apgsearch with this rule... it simply doesn't work. It takes up to 10-30 minutes to process just one soup on a 4-core 4 GHz. Apgluxe crashes after 8-10 soups. Didn't try with the Python version yet, but even if does work, it'll probably take half a year to produce one 10000-soup haul... and still would not work! I've logged about 40 processed soups and found out that Apgsearch doesn't know how to filter out pseudo still-lifes and groups of close, but independent gliders. Hauls submitted for Generations rules on Catagolue suffer from the same problem: Many recorded "objects" are groups of independent objects put next to each other, sometimes even separated by 5 of more cells.
I didn't check the Agpsearch's code yet, but I suspect that the first problem has to do with the abundance of natural breeders, while the filtering problem is due to having more than 2 states and, in the case of this rules family, with having different-looking color flavors and phases of the same object.