For a certain gas the ratio of specific heat is 1.5, for this has:

specific heat ratio = γ=CpCv =1.5   ⇒ Cp=1.5Cvdifference of specific heats is Cp-Cv=RJ  substitute given values in above equation  and Cp-Cp1.5=RJor Cp=RJ(1−11.5)=3RJ.