To solve the equation \( (iv) y + ix = 12 - 7i \) for \( x + y \), we start by separating the real and imaginary parts.
The given equation can be rewritten as:
\[ y + ix = 12 - 7i \]
We compare the real part and the imaginary part from both sides:
- The real part on the left side is \( y \).
- The imaginary part on the left side is \( x \).
From the right side, we identify:
- The real part is \( 12 \).
- The imaginary part is \( -7 \).
This gives us the following system of equations:
- \( y = 12 \)
- \( x = -7 \)
Now, we can find \( x + y \):
\[ x + y = -7 + 12 = 5 \]
Thus, the solution is:
\[ \boxed{5} \]