Please help. I don't even know where to start on this.

It's Sweden's nuclear power generation data

x=5 y=25.3
x=10 y=55.8
x=15 y=65.2
x=20 y=66.5

a) find a natural log regression equation for the data
b) predict when Sweden's nuclear power generation will reach 85 billion kilowatt-hour

I think I can figure out b but I have no clue where to even begin on part a and I've been searching for hours.