This problem involves two different types of measurements: meters and centimeters.
We want all the measurements in a problem to be same.
So let's first convert 400 cm to meters.
We know 100 cm = 1 meter (this should be memorized) so 400 cm must be 4 meters
This is an algebra problem so in any algebra problem you always want to write out the algebraic equation.
The algebraic equation for area of a rectangle is length times width or
A = L x W.
The problem gives us the area: A= 12 meters squared
And the width: W= 4 meters
Let's rearrange the equation to find and isolate length: L = A / W
Now we just plug in: L = 12 meters / 4 meters
12 divided by 4 = ?