How to find the debroglie wavelength?

You don't give any units for the speed; is it meters per second? If so, it is not great
enough to be relativistic, so we can use classical physical considerations.
The deBroglie wavelength of a moving body is found by dividing Planck's constant
(h) by the body's momentum, which is the product of its mass and velocity.
So the wavelength w is found by: w = h/p = h/mv and h = 6.625 * 10**-34 J-sec.
From my physics book I find the mass of a helium atom as 4.03 amu, or
6.69 * 10**-27 kilogram, so if the velocity is 8.5 * 10**5 meters per second, the
momentum (mv) is (6.69 * 10**-27)(8.5 * 10**5) = 5.686 * 10**-21 kg-m/sec.
The wavelength w = h/mv = (6.625 * 10**-34 J-sec) / (5.686 * 10**-21 kg-m/sec)
or 1.165 * 10**-13 meters.
You don't give units for your value above, but this is the value I obtained.