How do i complete a square to get a perfect square?

Will give brainliest sum of arithmetic series (sigma notation) find the numerical answer to the summation given below. (image shown below)

Asocial networking site currently has 400,000 active members, and that figure is predicted to increase by 32% each year. the following expression represents the number of active members on the site after t years.

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie