Amplitude of an AM wave
The amplitude of the AM wave depends on the modulating signal, that's why it is called amplitude modulation. Taking your values, we have:


If the modulating signal is at its mininum (negative peak), the AM wave amplitude is 25 V (swings between 25 and -25 V). 
If the modulating signal is at its maximum (positive peak), the AM wave amplitude is 75 V (swings between 75 and -75 V). 
If the modulating signal is 0, the AM wave amplitude is 50 V (swings between 50 and -50 V).


One thing to note is that the modulating signal is a very slow time-varying signal compared to the carrier frequency.