Okay, sir.
In the following commit, I have provided a crude solution for converting arcsecsq to pixnum. Whenever you got some time, I would love to hear your feedback so that I can improve the code(^_^).
 
The commit can be found at the following link
https://github.com/Bharat-bhandari/gnuastro1/compare/task%2316222?expand=1

Thanks Mohammad sir, this clears most of my doubts.

Now,for converting arcsecsq to number of pixels, I am using this formula. number of pixels = (arcseconds squared) / (pixel scale)^2

Added this code in lib/units.c.

.
double
gal_units_arcsecsq_to_pixnum(double area_arcsec2, double pixelscale)
{
  return area_arcsec2/pow(pixelscale,2);
}
.

required code will going to update in lib/arithmetic.c, gnuastro/arithmetic.h and gnuastro/units.h

Am I going in right direction?

PS :- I apologize for asking so many questions. Since it's my first open source contribution, so going a bit more comprehensive (^_^).

Hello Mohammad Sir👋,
I'm trying to solve this issue, but a bit confused here
Specifically, where can the related code be found in path/to/bin/arithmetic, bin/convertt, or bin/fits..  As the task is similar to unit convertion operator.
A brief suggestion, or any tips about the task would be really helpful sir😀

It would be very useful to have operators for converting arcsec^2 to number of pixels, and vice-versa. Based on an input WCS. We currently have the necessary operators to convert magnitudes to counts and vice-versa, but we don't have any for converting common area units.

For example:

$ astarithmetic 1 arcsecsq-to-pixnum --wcsfile=image.fits

This will be useful when we need to convert mag/arcsec^2 into image units: counts/pixel. In fact since this is a common conversion, we can have a high-level operator for it, maybe something like this:

$ astarithmetic 25 surfbright-to-countsperpix --wcsfile=image.fits

Of course, if an image with WCS is already present in the Arithmetic command, there is no need to use '--wcsfile'.

These can also be available for column arithmetic in Table.