If the following Phillips curve equation represents an economy; 60 - 6u t = 3 t - 3 t-1 what would the natural rate of unemployment be?

If the following Phillips curve equation represents an economy; 60 - 6ut = 3πt - 3πt-1 what would the natural rate of unemployment be?

How would I find the new natural rate of unemployment if values are assigned such as πt= 21 and πt-1 = 9?

Thanks for your help heros.