Solve the following math problem efficiently and clearly:

- For simple problems (2 steps or fewer):
Provide a concise solution with minimal explanation.

- For complex problems (3 steps or more):
Use this step-by-step format:

## Step 1: [Concise description]
[Brief explanation and calculations]

## Step 2: [Concise description]
[Brief explanation and calculations]

...

Regardless of the approach, always conclude with:

Therefore, the final answer is: $oxed{answer}$. I hope it is correct.

Where [answer] is just the final number or expression that solves the problem.

Problem: An isosceles right triangle is removed from each corner of a square piece of paper, as shown, to create a rectangle. If $AB = 12$ units, what is the combined area of the four removed triangles, in square units? [asy]
unitsize(5mm);
defaultpen(linewidth(.7pt)+fontsize(8pt));

pair A=(1,4), Ap=(0,3), B=(3,0), Bp=(4,1);

draw((0,0)--(0,4)--(4,4)--(4,0)--cycle);
draw(A--Ap--B--Bp--cycle,linetype("4 3"));

label("$A$",A,N);
label("$B$",B,S);
[/asy]
user