How can I present exercises in Video-conference mode? How can I write equations?

There are several options. In any case, you should test them with some friend beforehand, especially to check that what you write is readable. You should not wait for the exam or class to use them for the first time!

1) If you have a tablet or a laptop with an electronic pen, then it's easy. Just make sure you are familiar with the software

2) Simple options:
Write on a piece of paper and either
2a) Connect to zoom/webex a second time with your cellphone and use the cellphone camera to film the paper. Use the back camera, so you see on the screen how the image is. You may build a simple callphone holder, or simply hold it with one hand.
IMPORTANT: in this case you should connect your audio via your cellphone and not your laptop. Otherwise it may be difficult for some of your colleagues to see the cellphone film, due to the "speaker view" setup.
2b) Turn the piece of paper upside down and film it with the laptop camera by tilting the screen a bit. In some applications it is possible to turn the camera image by 180 degrees.

3) Make a Pocket-document-camera see also here for a quick version