添加两个长度不等的数据帧的列并填充缺失值

I am trying to add the columns of two dataframes. The dataframes comprise the three year data from 2010 to 2012. While downloading I found that the two datasets are of unequal length. It means that some of the data is missing. Now, I want to make these datasets of equal length and fill in the missing data first using linear interpolation. After that I could then combine the datasets. I am trying to add the columns of two dataframe