A bedroom is 6 m long, 4 m wide, and 2.8 m high. It has a window 1.2 m wide and 1.5 m high, and a door 0.9 m wide and 2.1 m high. Calculate the area to be painted.

The bedroom has a length of $6 \text{ m}$, a width of $4 \text{ m}$, and a height of $2.8 \text{ m}$.
The total area of the four walls is given by the perimeter multiplied by the height.

$$\text{Perimeter} = 2 \times (\text{length} + \text{width})$$ $$\text{Perimeter} = 2 \times (6 \text{ m} + 4 \text{ m}) = 2 \times 10 \text{ m} = 20 \text{ m}$$ $$\text{Area of walls} = \text{Perimeter} \times \text{height}$$ $$\text{Area of walls} = 20 \text{ m} \times 2.8 \text{ m} = 56 \text{ m}^2$$

The window has a width of $1.2 \text{ m}$ and a height of $1.5 \text{ m}$.

$$\text{Area of window} = \text{width} \times \text{height}$$ $$\text{Area of window} = 1.2 \text{ m} \times 1.5 \text{ m} = 1.8 \text{ m}^2$$

The door has a width of $0.9 \text{ m}$ and a height of $2.1 \text{ m}$.

$$\text{Area of door} = \text{width} \times \text{height}$$ $$\text{Area of door} = 0.9 \text{ m} \times 2.1 \text{ m} = 1.89 \text{ m}^2$$

The area to be painted is the total wall area minus the areas of the window and the door.

$$\text{Area to be painted} = \text{Area of walls} - \text{Area of window} - \text{Area of door}$$ $$\text{Area to be painted} = 56 \text{ m}^2 - 1.8 \text{ m}^2 - 1.89 \text{ m}^2$$ $$\text{Area to be painted} = 56 \text{ m}^2 - 3.69 \text{ m}^2 = 52.31 \text{ m}^2$$

1 litre of paint covers $9 \text{ m}^2$.

$$\text{Litres needed} = \frac{\text{Area to be painted}}{\text{Coverage per litre}}$$ $$\text{Litres needed} = \frac{52.31 \text{ m}^2}{9 \text{ m}^2/\text{litre}} \approx 5.8122 \text{ litres}$$

Paint is sold in 5-litre cans only. Since Thabiso cannot buy a fraction of a can, he must round up to ensure he has enough paint.

$$\text{Number of cans} = \frac{\text{Litres needed}}{\text{Litres per can}}$$ $$\text{Number of cans} = \frac{5.8122 \text{ litres}}{5 \text{ litres/can}} \approx 1.1624 \text{ cans}$$

Rounding up to the nearest whole can:

$$\text{Number of cans} = \lceil 1.1624 \rceil = 2 \text{ cans}$$

Thabiso needs to buy $\boxed{\text{2 cans}}$ of paint.