After 8 years, Greg's account earned $1800 in interest. If the interest rate (in decimal form) is 0.08, how much did Greg initially invest?

We can solve this equation  [ assuming yearly compounding ]

P + 1800   =   P (1 +  .08)^8    where P is the amount  originally invested

Rearrange as

1800  = P [ (1 + .08)^8  - 1]      divide both sides by [ (1 + .08)^8  - 1]

1800 / [ (1 + .08)^8  - 1]    =   P   =  about  $2115.38