Which is a better deal: 6x340ml cans for R36 or 3x2L bottles for R45?

A pack contains $6 \times 340 \text{ml}$ cans for R36.
Total volume in one pack of cans:

$$6 \times 340 \text{ ml} = 2040 \text{ ml}$$

Convert milliliters to liters:

$$2040 \text{ ml} = \frac{2040}{1000} \text{ L} = 2.04 \text{ L}$$

Cost per liter for cans:

$$\frac{\text{R}36}{2.04 \text{ L}} \approx \text{R}17.65 \text{ per liter}$$

A pack contains $3 \times 2 \text{ liter}$ bottles for R45.00.
Total volume in one pack of bottles:

$$3 \times 2 \text{ L} = 6 \text{ L}$$

Cost per liter for bottles:

$$\frac{\text{R}45.00}{6 \text{ L}} = \text{R}7.50 \text{ per liter}$$

Cost per liter for cans: R17.65
Cost per liter for bottles: R7.50

Since R7.50 is less than R17.65, the $2 \text{ liter}$ bottles offer a better value per liter.

The best option to pay is the 3 x 2 liters @ R45.00 per pack of 3.