For an amplitude modulated wave, the maximum amplitude is found to be 10V  while the minimum amplitude is found to be  2V. The modulation index  μ is

since the amplitude modulated wave is given by  (Ac+Amsinωmt)cosωct, the maximum amplitude is  M1=Ac+Am.While the minimum amplitude is M2=Ac−Am . Hence the modulation Index isμ=AmAc=M1-M2M1 +M2=10−210+2=812=23So μ=23