The image contains two questions. The first, a chemistry question, is incomplete as the chemical reaction is not provided. The second is a mathematics question.

$$12 = 2 \times 2 \times 3 = 2^2 \times 3$$ $$20 = 2 \times 2 \times 5 = 2^2 \times 5$$

The common prime factors are $2^2$.

$$\text{GCD}(12, 20) = 2^2 = 4$$

The side length of the maximum size square tile is $4 \text{ m}$.

The question is cut off, but assuming it asks for the side length of the maximum size tile:
The side length of the maximum size square tile is $\boxed{\text{4 m}}$.

Regarding the chemistry question "(b) write a net ionic equation for the chemical reaction", the chemical reaction itself is not provided in the image, so a net ionic equation cannot be written.