You will have to post a lot more information to get a useful response. I'm guessing this is a homework problem, so post all the details of the problem and someone might give you an ideal how to solve it.
You are probably calculating the gate width. If this is an analog circuit, you will probably be trying to achieve a specific Id current for a specified Vov (or Vgs/Vt combination). The Id equation (1/2 k'n (W/L)(Vov)^2 possibly with the channel modulation term added is usually the right equation to use. If you are targeting a specific gain or transconductance then the technique depends on the problem givens.
