Which is bigger 6 km or 54,000 mm < > =

So,

We need to know the conversion factor between km (kilometer) and mm (millimeter).
We know that a kilometer is 1000 meters, and that a meter is 1000 millimeters.

Therefore, 6 km is 6000 m, and 6000 m is 6,000,000 mm.  We can immediately see that
6,000,000 mm > 54,000 mm

Therefore, 6 km is (much) larger than 54,000 mm.

The expression of this would be 3.75x+y= 23