For image formation due to M1,

 

1/f=1/u+1/v

 

Here f=20cm , u=30cm

 

So v=60cm

 

Let distance of S from M2 be x.

 

The image due to M1 acts as virtual object and this virtual object will be at a distance =60-(30+x)=30-x

 

For M2 f= -20cm , u= -(30-x)=x-30 and v= x

 

So using 1/f=1/u+1/v

 

We get x=20cm

 

a)So distance between two mirrors is 30+20 =50cm

 

b)Use 1/f=1/u+1/v

 

Here f= -20cm , u=20cm

So v= -10cm