Dell Inspiron 15 3521 line in microphone not working

Hello there. I have a Dell Inspiron 15 3521 laptop which has a combo jack

Hello. You cannot use a stand alone mic in a Dell combo jack. The kind of microphone it accepts is the kind built into a combo headset (a Smart Phone headset). Here is a picture of such a headset. Notice that the plug has 4 segments instead of the normal 3 segments. It combines headphones and mic into a single plug.

If you have a normal mic with a plug with 3 segments, you would have to obtain an adapter to use it with a Dell combo jack. The adapter is called Headset Buddy and it looks like this. Notice the 4 segmented plug.

You're welcome.

Here is a picture of a different Headset Buddy that looks similar to the one above, but notice that its plug has 3 segments. This one is the wrong Headset Buddy so don't get it, and if your mic plug is like this it won't work in the combo jack because the contacts won't mate. Note that regular headphones with normal 3 segmented plugs do work in the combo jacks. It is just the mics that don't without an adapter.