As you have asked about the answer key of the Railway Recruitment Board (RRB) Guwahati of Junior Engineer (JE) exam, I want to tell you that it has not been released yet.

However, you can check for it regularly from its website.
To check it, you can visit RRB Guwahati website the page open look like

Here you will find the information about the answer key when available.

Here I found following information:

Provisional Panel of NTPC-UG SRD PWD against CEN 02-2015
Final Document Verification for Absentee Candidates - CEN 01-2015
Provisional Panel of SSE against CEN 02-2014
Provisional Panel of JE against CEN 02-2014
Mock Test and Important Information to Candidates for CEN-03/2015-NTPC (Graduate) Online Examination
Download Call Letter for Document Verification CEN 01/2015 (JE & SSE)
Rectified Provisional result of Online Examination (JE and SSE) against CEN 01-2015
Call Letter for Verification of Documents and Genuineness of Candidature for CEN 02/2015 SRD for PwD- NTPC (UG) Categories