The answer would be 1,220,000mm.
You can do this if you convert all the measures into one unit. Let us convert all into km.
The first one is already in km, so we do not need to covert it.
Let's start with converting 1,220,000mm to km.
There are 1, 000,000 mm in one km.
So there are 1.22km in 1,220,000mm.
Next, we have 120m. There are 1,000m in 1km.
There are 0.12km in 120m
Now you can see that 1,220,000mm is the longest.
You can do this if you convert all the measures into one unit. Let us convert all into km.
The first one is already in km, so we do not need to covert it.
Let's start with converting 1,220,000mm to km.
There are 1, 000,000 mm in one km.
So there are 1.22km in 1,220,000mm.
Next, we have 120m. There are 1,000m in 1km.
There are 0.12km in 120m
Now you can see that 1,220,000mm is the longest.