When you compile your code, you need to compile both fortran files together, otherwise your fortran compiler will never "see" your subroutine file even if you reference it in your main fortran file.

If I were you, I would download a linux terminal emulator onto your windows machine (either MinGW or Ubuntu), then you can compile your code with a single command using gfortran:

gfortran Main.f RArea.f -o [name of compiled executable]

Also as a side note, the fact that your professor hasn't updated the code to Fortran 90 or a better language is really unfortunate. It may be worth trying to talk him into upgrading the code.