Step 1: Understanding the Concept:
If a line passes through a given point, the coordinates of that point must satisfy the equation of the line.
Step 2: Detailed Explanation:
The given equation of the line is \( x - y = 7 \).
The line passes through the point \( (4, a) \).
Substituting \( x = 4 \) and \( y = a \) into the equation:
\[ 4 - a = 7 \]
Solving for \( a \):
\[ -a = 7 - 4 \]
\[ -a = 3 \]
\[ a = -3 \]
We can verify with the other point \( (3, -4) \): \( 3 - (-4) = 3 + 4 = 7 \), which is correct.
Step 3: Final Answer:
The value of $a$ is -3.