To find the total cost of painting the rectangular wall, we first need to calculate the area of the wall in square meters. The area \( A \) of a rectangle is given by the formula:
\[ A = \text{length} \times \text{height} \]
In this case, the length of the wall is 6 meters and the height is 3 meters.
Calculating the area:
\[ A = 6 , \text{m} \times 3 , \text{m} = 18 , \text{m}^2 \]
Now, the painter charges R20 per square meter. To find the total cost \( C \), we multiply the area by the cost per square meter:
\[ C = A \times \text{cost per square meter} \] \[ C = 18 , \text{m}^2 \times R20/\text{m}^2 = R360 \]
Thus, the total cost of painting the wall is R360.