there are 2 things on this one

1. Since this is a linear DE the whole problem can be solved easier using an integrating factor

is set up perfectly for this

with the integrating factor e^(.4*t^2)

However suppose you want to separate variables

2.first you were ok up to

To integrate the left use u = 2 - 0.8y

you tried to write as dy/2 -dy/(.8y) which is not true