Answer:
The expression means equivalently,
3.9 centuries, or
39 decades, or
390 years
Explanation:
Since 1 century has 10 decades. So,
6 decades mean 0.6 centuries
Hence,
2 centuries 6 decades + 1.3 century means,
2.6 centuries + 1.3 centuries
= 3.9 centuries
or,
39 decades
or,
since 1 decade has 10 years and 1 century has 100 years,
so 3.9 centuries means 390 years