P1V1=nRT

P2V2=n'RT

• n=dV1/M1 (d=density) ..............(1)
• n'=d'V2/28
• d=2*d'

(1) becomes

n=2*d'V1/M1;

P1V1=(2*d'V1/M1)*RT .................(2)   {V1 gets cancelled}

P2V2=(d'V2/28)*RT.......................(3)   {V2 gets cancelled}

taking  (2)/(3):

P1/P2=(2/M1)/(1/28)

->2/4=56/M1

->M1=112g mol^(-1)

pm=drt

p1= 2

p2= 4   d1= 2d2

m=56

We know 

PV=nRT ,

d1=d2,

P1×M1=P2×M2,

2×2m=4×28

m1=28 g mol^-1