I would first try simplfying it, so you would get x^2(x+1)=392. Then i would factor 392 and see if there are any numbers that you can imedietly see that would fit. So, 392=2^3 * 7^2, we can try to say that 7 is x, from there we get 7^2(7+1). 7+1=8=2^3 so from here x=7.