Hi there. Hope you understand english. If i got it rught, they ask you to identify the chemical formula of the substance, right? Ok, here's the computation:
Let the primitive formula of the substance as the following: C_xH_y. 
x:y=w©/A_r©: w(H)/A_r(H)=85.71/12:14.29/1=7.1425:14.29=1:2. That means that the elementary formula of the organic substance is CH₂
To get its authentic formula, we need to know its density which is given. D[(CH₂)_z ]=M[(CH₂)_z ]/M(CO_2). Therefore, the molecular weight of the substance : 14*z=44*1.91; z= 6.
That points to the authentic brutto- formula: C₆H₁₂  Possible isomers of the substance are as following
a) CH₃-CH=CH-CH₂-CH₂-CH₃ methyl-n-propyl - ethylene
b) CH₂=CH-(CH₂)₂-CH₃ n-propyl-ethylene
c) CH₃-CH₂-CH=CH-CH₂-CH₃ diethylethylene