Problem 3: Set up an inequality for a farmer who needs at least 300 kg of maize and has already harvested y kg. Problem 4: Convert 126 km/h to m/s.

Let $y$ be the amount of maize already harvested (in kg).
Let $x$ be the additional amount of maize the farmer must harvest (in kg).

The farmer needs at least $300 \text{ kg}$ of maize. This means the total amount harvested ($y + x$) must be greater than or equal to $300 \text{ kg}$.

The inequality showing how much more maize the farmer must harvest is $\boxed{x \ge 300 - y}$.

: Convert speed from km/h to m/s

The speed of the train is $126 \text{ km/h}$.

We know that $1 \text{ km} = 1000 \text{ m}$.
So, $126 \text{ km} = 126 \times 1000 \text{ m} = 126000 \text{ m}$.

We know that $1 \text{ hour} = 60 \text{ minutes}$ and $1 \text{ minute} = 60 \text{ seconds}$.
So, $1 \text{ hour} = 60 \times 60 \text{ seconds} = 3600 \text{ seconds}$.

To convert $126 \text{ km/h}$ to $\text{m/s}$, we multiply by the conversion factors:

The speed of the train in $\text{m/s}$ is $\boxed{35 \text{ m/s}}$.