In this instance, the "x" means "times" or "multipied by". When dimensional indicators (numbers) are inserted for "width" and "heighth", a perimeter value may be easily calculated. Mathematically...
You usex-ray as a verb just as it is. Depending on past, present and future tense, it would change.
Example: I will x-ray the fish tomorrow.
I x-rayed the fish yesterday.
I am x-raying the fish in 2...
