You have the following system of equations:

1. $x = y$
2. $4x + 9y = -39$

Let's solve it step by step:

Step 1: Substitute $x = y$ into the second equation.

Since $x = y$, replace $x$ with $y$ in the second equation:

$4y + 9y = -39$

Step 2: Combine like terms.

$13y = -39$

Step 3: Solve for $y$.

$y = \frac{-39}{13} = -3$

Step 4: Since $x = y$, we also have:

$x = -3$

Final Solution:

$x = -3$ and $y = -3$.

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Would you like more details on any of the steps or have any other questions about this problem?

Here are 5 related questions:

1. How would you solve a system of linear equations using substitution?
2. What happens if the system of equations is inconsistent?
3. How do you graph the solution to a system of equations like this?
4. How can you solve systems of equations using matrices?
5. What is the significance of having the same value for $x$ and $y$ in a system of equations?

Tip: When solving systems of linear equations, always check the equations for any opportunities to simplify or substitute values early to make calculations easier.