Concept:
A square formed by vertical and horizontal lines will have its center at the midpoint of the boundary lines. The inscribed circle shares the same center.
Step 1: Solve given equations to find boundary lines.
For $x$:
\[
x^2 - 8x + 12 = 0
\]
\[
(x-2)(x-6) = 0
\]
\[
x = 2, x = 6
\]
For $y$:
\[
y^2 - 14y + 45 = 0
\]
\[
(y-5)(y-9) = 0
\]
\[
y = 5, y = 9
\]
Step 2: Identify square boundaries.
The square is bounded by:
\[
x = 2, x = 6, y = 5, y = 9
\]
Step 3: Find center of the square.
\[
a = \frac{2 + 6}{2} = 4
\]
\[
b = \frac{5 + 9}{2} = 7
\]
Step 4: Compute required value.
\[
a + b = 4 + 7 = 11
\]
Final Answer:
\[
\boxed{11}
\]