The sum of x² plus x² is 2x². The problem requires adding one x squared to one more x squared, which equals two x squared. Substituting 3 for x, 3² + 3² = 2(3²), or 9 + 9 = 18.
Each of the variables is raised to the same power, so they are like terms. In the case of one of the variables being squared and the other cubed, no like terms exist. Since this problem only has like terms, the problem is solved simply by adding the coefficients together. The understood "ones" in front of the variables are the coefficients in this problem.