First, that is not a concentration dimension. (.04 units)
 But I will assume that you are asking when will C(x) = .04
  
 .04 = .04t / (t^2-2)     Multiply both sides by  t^2-2
  
 .04t^2 -.08 =.04t
 .04t^2-.04t-.08=0     Multiply both sides by 100 (to make it easier to work with it)
 4t^2 - 4t - 8 = 0        Now divide by 4
 t^2-t-2 = 0
 (t-2)(t+1) = 0     means  either t=2  or  -1 hours    -1 hours is not possible, so the only REAL answer is t=2 hours