Students learn to add or subtract terms that involve variables by combining like terms. Like terms are terms with exactly the same variable factors. For example, 5x and 3x are like terms. However, 7x and 9y are not like terms, and neither are 8x and 8x^2. In the problem 2x + 3x + 4, only the 2x and 3x can be combined, because they are like terms, so the expression can be simplified to 5x + 4. Students also learn that the number in front of the variable is called the coefficient, and when there is no coefficient in front of the variable the coefficient is understood to be 1.