To solve this problem, we need to determine the value of x that makes the function equal to -13 when x is 5.
From the given function, we can see that when x is greater than or equal to -3, the function is defined as 7-4x.
So, if x is 5, we use the second part of the function:
f(5) = 7 - 4(5)
f(5) = 7 - 20
f(5) = -13
Therefore, the value of the unknown variable x is 5.
The answer is: 5
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Let \[f(x) =
\begin{cases}
3x + X &\text{if }x<-3, \\
7-4x&\text{if }x\ge -3.
\end{cases}
\]Find $f(5)$. The answer is -13. What is the value of unknown variable X?

### Answer:
