To solve this, let's break it down:

• Outer square's area: The side length is 12 units, so the area of the outer square is: $\text{Area of outer square} = 12 \times 12 = 144 \text{ square units}$

• 4 triangles' areas: Each triangle has an area of 18 square units, so the total area of the 4 triangles is: $\text{Total area of 4 triangles} = 4 \times 18 = 72 \text{ square units}$

Now, using the given equation:

$\text{Area of outer square} - \text{Sum of the areas of the 4 triangles} = \text{Area of inner square}$ Substitute the known values: $144 - 72 = \text{Area of inner square}$

So, the area of the inner square is: $\text{Area of inner square} = 72 \text{ square units}$

The final equation is: $144 - 72 = 72$

---

Would you like more details or clarification on any part? Here are 5 related questions to consider:

1. How does the area of a square relate to its side length?
2. How can you calculate the area of a triangle when given the base and height?
3. What geometric properties are important when working with tilted squares and intersections?
4. How do you find the area of a figure when it is enclosed within another shape?
5. How can the concept of symmetry help in solving geometric problems like this?

Tip: Remember, the area of a square is calculated as side length squared, and when dealing with intersections and enclosed figures, subtracting areas is key to finding what's left.