@Xiggi If this were for the current SAT I would agree with you completely. Since this is for the new SAT, my feeling is that it is not a certainty that a single operation will produce the answer right away. (In this problem it does, but in another similar problem it might not.)
I’m not saying not to use the method you described for this particular problem - it certainly works here.
But I think for the new test advanced students should practice the standard methods as well - elimination, Guass-Jordan reduction, substitution, and various graphing calculator methods.