Calculating Peak-to-Peak Amplitude of Out-of-Phase Sine Waves

[Sloan650]

Two sine waves have equal peak to peak amplitudes 81 but are out of phase by 90 degrees. What is the peak to peak amplitude of the resultant wave.


I have no idea how to start this question, could someone give me guidance?

 

[sandy.bridge]

Graph
$$40.5sin(\omega t-\frac{\pi}{2})$$ and $$40.5sin(\omega t)$$
The points on the graph where these functions intersect are where the resultant will have its maximum (and minimum) amplitudes.