According to Kepler's Law of period T2∝r3
where T= period of revolution, r= mean distance from sun
T21T22=r31r32;T1T2=(r1r2)3/2.
Let T2 be the period of revolution of earth around sun and r2 be the mean distance between earth and sun.
Given: (r1/r2)=5.2 and T2=365 days (one year).
Hence, T1=T2(r1r2)3/2=1(5.2)3/2=11.85 years.