Hey there!:
Wavelength λ = 0.650 fm = 0.650* 10⁻¹⁵ m (1 fm = 10⁻¹⁵ m)
Speed v = 129 m/s⁻¹
Planck constant h = 6.626 * 10⁻³⁴ J.s
De Broglie equation: λ = h/mv
Mass m = h/λv
= 6.626 * 10⁻³⁴/ ( 0.650* 10⁻¹⁵ * 129)
= 6.626*10⁻³⁴ / 8.385*10⁻¹⁴
= 7.902*10⁻²¹ Kg
Hope that helps!
