Simple Harmonic Motion on a moving platform

I had my physics midterm today and I totally blanked out. I want to know how to solve it for next time.

So In the picture, there are two springs connected to the mass on a platform.
a) if the platform is at rest, find the angular frequency

the expression for angular frequency is :ω=√(k/m), so I just added the two k's and plugged it into k and got :ω=√((k1+k2)/m). Is this correct?

b)The platform then started to move in SHM with the equation d(t)=D*cos10∏t. We had to find the amplitude of mass M. This is where I tripped up and don't know how to solve it. Any ideas?
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hi chrisy2012!
 Quote by chrisy2012 the expression for angular frequency is :ω=√(k/m), so I just added the two k's and plugged it into k and got :ω=√((k1+k2)/m). Is this correct?
maybe!
how would you prove it?
 b)The platform then started to move in SHM with the equation d(t)=D*cos10∏t.
find the positions of the two endpoints as functions of t, and that will help you find the distances to multiply by the spring constants