I filled amended tax return 1040x w/8936 for qualified plug in electric drive motor vehicle credit. I received back the 1040x w/8090-a asking to support line 7 on 8936

When you mailed in the 1040X did you include the form 8936? 

If so then send the supporting docs that you used to make the line 7 entry which may include the form 4562 and it's worksheet.  

Line 7

Enter any section 179 expense deduction you claimed for the vehicle from Part I of Form 4562, Depreciation and Amortization.

Yes, I did include form 8936. So I have to include another form 4562?

Not another ... the one in the return you already have since an amount was listed on line 7 of the 8936.  Seems like you didn't send it with the rest of the return because you didn't realize it was required.