If two waves have the same amplitude, frequency, and direction, but are out of phase by ϕ degrees, then the amplitude of the resultant waves is given by AR=2Acos(2Δϕ)
Here, A1=A2=AR=8cm
So, cos(2Δϕ)=21
Or phase difference between the individual waves is 2Δϕ=60∘⇒Δϕ=120∘.