braked to let someone merge and got honked at

New driver dilemma! Was in the right lane going speed limit when someone needed to merge from the on-ramp. We were side by side, so I braked to let them in. Car behind me honked like crazy! Did I mess up? Should I have maintained speed and let the merger figure it out? Highway etiquette is confusing! #newdriver #highwayetiquette #drivingtips #mergeproblems