I grabbed Desmos, and showed students that 2x and 3x could be graphed using y = 2x and y = 3x. Two lines...so, what happens when you add two lines? You just get a different line...but it is still a line. The students predicted a shift, but instead the line just got steeper.
Then we graphed y = 5x^2 and saw the parabola. Quite clearly students saw that 2x + 3x cannot be 5x^2 because you can't add two lines and get a parabola. However, they also saw that 2x + 3x = 5x^2 was sometimes true...when the graphs intersect. We substituted x = 0 and x = 1 to show that was true.