Question I got a computer, but dosent know the motherboard

The width of the motherboard seems to conform to ITX size.
But, the depth of the motherboard is more like eatx depth.
Here is a link to ATX specs and dimensions:
By measuring your motherboard and comparing to a prospective case, you should be able to find something that fits.
The dimensions are 13.8 x 13.4 x 6.4 inches (350.5 x 340.4 x 162.6mm)

The motherboard is proprietary and the extension is designed to mate with the front ports of the case.
If you were to mount the motherboard in a EATX case which is deeper, you would not be able to mate the front panel outputs to any other case.
If you want a better case, the easiest thing to do is to buy a replacement motherboard to go with the new case.
Most any lga1151 300 series will do for your i7-8700 processor.
Here is an example for $60.


The motherboard may have some proprietary connectors/headers, meaning that even if you fit it into another case it won't be able to connect to the case properly.

I would just buy an lga1151 board like the one geofelt linked if you are planning to swap cases.

Another consideration with a case swap could be the PSU and cable length.