At Max eff. the core losses are equal to copper losses. Start from assuming core losses to be equal to x. You must each the answer. Okay I will do it.
Let I be the full load current
At max eff (.45I)**2 X R=x
This gives me Full load losses= I**2 X R=4.94x
Now total losses at full; load= I**2 X R +x= 4.94x+x=5.94x=2000, gives me x= 336.8 and that is the answer.