To solve the problem, we need to address parts (a) and (b) as described:

### Part (a): Calculating the Area of the Land
The area can be calculated by breaking down the shape into two rectangles:

1. The larger rectangle:
- Width = 6 m, Height = 4 m
- Area = 6 m × 4 m = 24 m²

2. The smaller rectangle (cut-out):
- Width = 2 m, Height = 1 m
- Area = 2 m × 1 m = 2 m²

Now, subtract the area of the smaller rectangle from the area of the larger rectangle:

\[
\text{Total Area} = 24 \text{ m}² - 2 \text{ m}² = 22 \text{ m}²
\]

### Part (b): Cost Calculation for Planting Flowers
Cô Lan plans to plant 4 flower pots in the rectangular area, with the cost for each pot being 70,000 VND.

1. Total number of pots = 4
2. Cost per pot = 70,000 VND

Total cost:

\[
\text{Total Cost} = 4 \times 70,000 \text{ VND} = 280,000 \text{ VND}
\]

### Summary of Results
- **Area of the land:** 22 m²
- **Total cost for planting flowers:** 280,000 VND