91Ó°ÊÓ

Substitution is a useful method in algebra to make expressions easier to evaluate. It's particularly handy when working with algebraic expressions that involve variables. In our problem, the expression \(-x-y\) is given where we know the values for \(x\) and \(y\) are \(-7\) and \(-9\) respectively.
To apply substitution correctly:

• Identify the variables in the expression.
• Replace these variables with their known values.

For the given expression \(-x-y\), we substitute \(x = -7\) and \(y = -9\). This tells us to replace \(x\) with \(-7\) and \(y\) with \(-9\), transforming it into \(-(-7) - (-9)\).
This method sets the stage for further steps such as simplification, which makes the expression much more manageable.

Simplification is the process of reducing an expression to its simplest form, and it makes solving algebraic problems much easier. In our expression, \(-(-7) - (-9)\), simplification involves resolving double negatives and understanding the basic properties of numbers.
First, recognize that negating a negative number results in a positive. Therefore, \(-(-7)\) simplifies to \(7\) and \(-(-9)\) simplifies to \(9\).
This turns our expression into \(7 + 9\).
Simplification not only makes calculation straightforward by reducing complexity but also helps in reducing errors, especially in longer expressions. In this exercise, simplifying negations helps us clearly see what operation to perform next.

After the substitution and simplification steps, the expression has been reduced to numbers we can easily work with: \(7 + 9\).
The addition process involves combining these simplified terms to find their sum. Here:

• Identify the numbers to add, which in this simplified expression are \(7\) and \(9\).
• Perform the arithmetic addition.

Adding \(7\) and \(9\) results in \(16\).
This final step completes the evaluation of the original algebraic expression after performing substitution and simplification. Understanding addition is key to solving problems and verifying the accuracy of your results.