To combine the expression –9a–4a, you add the coefficients -9 and -4 together to get -13a, as they are like terms with the same variable 'a'.
The expression given is –9a–4a. In this expression, we want to combine like terms. The term 'like terms' refers to terms that have the same variable raised to the same power. Here, both terms have the variable 'a' and they are both to the first power. Therefore, we can combine them by adding the coefficients (the numbers in front of the variables).
To combine –9a and –4a, you would add the coefficients -9 and -4 together. The process is similar to the numerical example where 2 minus negative 6 is written as 2+6 which equals 8. In this case, we are dealing with negative coefficients, so we keep their signs and add the values.
–9a–4a simplifies to –13a since -9 + -4 equals -13.