1. Find the area of the square base by squaring the side length (12 in this case): 12^2 = 144 square units.

2. Calculate the perimeter of the square base by multiplying the side length by 4: 4 * 12 = 48 units.

3. Plug the base area (144), perimeter (48), and slant height (18) into the formula to find the surface area:
Surface area = base area + 1/2 * perimeter * slant height
Surface area = 144 + 1/2 * 48 * 18
Surface area = 144 + 24 * 18
Surface area = 144 + 432
Surface area = 576 square units.

Therefore, the surface area of the square pyramid is 576 square units by following these steps.

Use a different word for squaring, and plugging in

1. Calculate the area of the square base by raising the side length to the power of 2 (12 in this case): 12^2 = 144 square units.

2. Determine the perimeter of the square base by multiplying the side length by 4: 4 * 12 = 48 units.

3. Substitute the base area (144), perimeter (48), and slant height (18) into the formula to find the surface area:
Surface area = base area + 1/2 * perimeter * slant height
Surface area = 144 + 1/2 * 48 * 18
Surface area = 144 + 24 * 18
Surface area = 144 + 432
Surface area = 576 square units.

Therefore, the surface area of the square pyramid is 576 square units by following these steps.