What is the mass of a rock that weights 49 newtons?

Remember that the Weight of any mass is

Weight = (mass) x (gravity)

and gravity on Earth is 9.8 m/s² .

Now just fill in the numbers we know:

49 Newtons = (mass) x (9.8 m/s²)

49 kg-m/s² = (mass) x (9.8 m/s²)

Divide each side by  (9.8 m/s²) , and we have

mass = (49 kg-m/s²) / (9.8 m/s²)

mass = (49 kg-m/s²) x (s²/9.8m)

mass = (49/9.8) x (kg-m-s²/s²-m)

mass = 5 kg

wavelength variability is the right one