Calculate the output 1dB compression point P1dB

Alright so attached is the question. Alright so i've been stuck on this problem for some time. here is what i understand from the question and how far i've solved it (Although i'm not sure if i'm right). So i'm given the output voltage. I know that the input voltage is given by: Vi = VoCos(wt). so i plugged this into the output voltage and got the following: Vo=10VoCos(wt) + 0.4Vo^2Cos^2(wt) + 0.66Vo^3Cos^3(wt). this gives me a1 = 10, a2 = 0.4 and a3 = 0.66. I also know that the gain compression can be calculated: G = a1+ (3/4)a3Vo^2. but i don't know what to do from here. I don't have the value of Vo. I'm stuck. Any help is highly appreciated.