"umol" stands for micromole. You can convert mole into micromole by multiplying a certain factor. For this case, 1 mol is known to be equal yo 1x10^6 umol. This is the conversion factor that will be multiplied to 3.5.
3.5 mol ( 1x10^6 umol/ 1 mol) = 3.5 x10^6 umol