how can i design this part (forming shoulder) with solidworks 2018

Okay, those are significantly different to the original posted. They're actually easier to produce CAD and manufacturing wise, so let me find some time to build a close approximation so I can send you something closer to what you're after.

The original for reference:

Probably would be best if you worked through it yourself, then asked about the parts you were having issues with.

The only difficult part on this model (the one on the left) is that it was designed to be formed as a sheet-metal part. Since you don't have distinct requirements, you can take a bit of liberty in design. The one on the right is a bit easier. I worked through the left part, just to see what the workflow might be... though not in SolidWorks.

Would be nice to see a top view, to see if the radius actually extends to the sharp corner' not a good corner to work with and didn't bother fixing mine when I got to it... but it would be nice to know what the actual geometry is in along the back of the model.

Any specific questions, just ask.

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