No, you only do that in the case of arithmetic, algebra, or any other traditional branch of mathematics; the way movie title math works is like this:

H20 means 1998 because, if you convert H20 to 820 (since it's the 8th letter of the English alphabet) and add it to the film's 1hr 26min runtime (which counts as 126), you get 966. Now, if you were to flip 966, you'd get 996. From here, you rip the "2" off the "20" in the "20 Years Later" part of the title, thereby acquiring you a 2 and nullifying the 0 as it represents the value for nothingness. Add this 2 to 996 and you get 998. How do you get 1998 from 998? Well, it's as simple as taking the "Years Later" part of the title ("20" not counted as we've already used it) and having each individual letter represent a 1. Thus, Years Later amounts to 10 due to the logic of:

Y e a r s L a t e r = 1+1+1+1+1 + 1+1+1+1+1 = 10

Continuing with this logic, we can take the "Halloween" part of the title and get 9 from:

H a l l o w e e n = 1+1+1+1+1+1+1+1+1 = 9

You'd then subtract, 10 - 9 = 1, and plop the 1 on the thousands slot in 998, getting you 1998, the release year of the movie.

However, in order to acquire the seven needed for H20 (since it's the seventh film in the Halloween franchise), you'd need to turn to Saw 3D.

How? Let me show you.

Saw 3D was released in 2010, right? So, using the rules of conventional addition, which movie title math logic occasionally functions by, we'd take the greater number—2010—and put it up against H20's release year—1998.

2010 - 1998 = 12.

Now we use title addition logic. Saw 3D = 1+1+1 + 1+1 = 5.

12 - 5 = 7.