The calculated distance (air line) between Saint-Quentin-en-Yvelines and Cergy is approximately 18 miles respectively 30 kilometers.
By car or train, the actual journey to Cergy is certainly longer, as only the direct route (as the crow flies) between Saint-Quentin-en-Yvelines and Cergy has been calculated here.
Air line (approximately)
30 kilometers
16 nautical miles