Final answer:
To add or subtract algebraic expressions, identical variables and exponents are required, as like terms with the same variable factors can be combined.
Step-by-step explanation:
In order to add or subtract algebraic expressions, you MUST have identical variables and exponents. This is because you can only combine like terms when they have the same variable factors and their exponents are the same. For instance, 2x and 5x can be added to get 7x, whereas 2x and 5y cannot be directly added since they represent different variables.
While adding or subtracting numbers in scientific notation, you must convert them to a form where all the exponents have the same value, which ensures you are combining like terms. Similarly, when multiplying or dividing numbers with exponents, there are specific rules related to the signs that come into play. However, these rules are different from those required for the addition and subtraction of algebraic expressions.