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: What is the area, in square units, of triangle $ABC$ in the figure shown if points $A$, $B$, $C$ and $D$ are coplanar, angle $D$ is a right angle, $AC = 13$, $AB = 15$ and $DC = 5$? [asy]
pair A, B, C, D;
A=(12,0);
D=(0,0);
C=(0,5);
B=(0,9);
draw(A--B--C--A--D--C);
draw((0,.5)--(.5,.5)--(.5,0));
label("$A$", A, dir(-45));
label("$B$", B, dir(135));
label("$C$", C, dir(180));
label("$D$", D, dir(-135));
[/asy]
user