First, take the molar mass of B (10.81g) and divide the value you've been given by the molar mass to get moles (5.4 / 10.81). So you have .5 moles of B and then you need to convert that to atoms. To do that you must use the constant 6.022X10^23 atoms per mole. Take your moles and multiply it by that constant. Your final answer should be (3.01X10^23) atoms.
Hope this helps!
