Displacement equation of second wave can be written as,y2=5(sinωt+3cosωt)=10(sinωt×21+23×cosωt)=10(sinωt×cos3π+sin3π×cosωt)
=10sin(ωt+3π)
As we can see, the phase difference between the waves is 0. Therefore, resultant amplitude will be sum of the amplitudes of the waves.
So A=A1+A2=10cm+10cm=20cm