To calculate the de Broglie wavelength for a particle, or for a tennis ball for that matter, just use the equation
#p = h/(lamda)#, where
#p# - the momentum of the atom;
#h# - Planck's constant - #6.626 * 10^(-34)"m"^(2)"kg s"^(-1)#
#lamda# - wavelength;
Momentum can be expressed as
#p = m* v#, where
#m# - the mass of the particle;
#v# - the speed of the particle.
So, starting with the electron that travels at 10% of the speed of light. The speed of light can be approximated to be
#c = 3 * 10^(8)"m/s "#, which means that the electron's speed will be
#v = 1/10 * c = 3 * 10^(7)"m/s"#
The mass of an electron is #m = 9.1094 * 10^(-31)"kg"#
Now plug your values into the main equation and solve for #lamda#
#p = h/(lamda) => m * v = h/(lamda) => lamda = h/(m * v)#
#lamda_"electron" = (6.626 * 10^(-34)"m"^(cancel(2))cancel("kg")cancel("s"^(-1)))/(9.1094 * 10^(-31)cancel("kg") * 3 * 10^(7)cancel("m")cancel("s"^(-1))#
#lamda_"electron" = color(green)(2.42 * 10^(-11)"m")#
Now for the tennis ball
#lamda_"tennis" = (6.626 * 10^(-34)"m"^(cancel(2))cancel("kg")cancel("s"^(-1)))/(55 * 10^(-3)cancel("kg") * 35cancel("m")cancel("s"^(-1))#
#lamda_"tennis" = color(green)(3.44 * 10^(-34)"m")#
