ברשותכם means 'in your possession', that is, 'you have'

Birshutchem

The sentence means "If you have a suitcase with wheels"

The word is made up of three parts:

ב-רשות-כם

ב       be - in

רְשׁוּת   reshut - possession

כם     -chem - your (masculine plural)

בְּ-רְשותכם  --->  בִּרְשותכם

be-reshut-chem contracts to birshut-chem because of the two shevas at the beginning (two-dot vowels under the ב and ר)

See case 3 of the grammar on this page if you want a better explanation of this contraction:

http://www.hebrew4christians.com/Grammar/Unit_Seven/Inseparable_Prepositions/inseparable_prepositions.html

I hope this helps