De or het sleephelling?

The correct Dutch article is:

de sleephelling

Rule Words ending in -ing: almost always de. See the rule

How to use sleephelling

the sleephellingde sleephelling
this sleephellingdeze sleephelling
that sleephellingdie sleephelling
our sleephellingonze sleephelling
your sleephellingjouw sleephelling
every sleephellingelke sleephelling
which sleephelling?welke sleephelling?
a big sleephelling / the big sleephellingeen grote sleephelling, de grote sleephelling