software installation - Failed to fetch http://ppa.launchpad.net

When I run

sudo apt-get update

I always get this output:

...

E: Failed to fetch http://ppa.launchpad.net/q-quark/equalx/ubuntu/dists/xenial/main/source/Sources  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead

How could I fix it?