If a boulder has a weight of 675,000 N what is it's mass

Weight = (mass) x (acceleration of gravity where the object is)

You didn't tell us WHERE the boulder is, so I have to assume that it's on Mars, where the acceleration of gravity is 3.71 m/s².

675,000 N = (mass) (3.71 m/s²)

Mass = (675,000 N) / (3.71 m/s²)

Mass = 181,941 kilograms

The same weight on Earth would suggest a mass of only 68,807 kg, so you can see how important it is to know where you are when you make your measurements.