Maximum efficiency of a heat engine

[JJones_86]

Homework Statement

Calculate the maximum possible efficiency of a heat engine that uses surface lake water at 19°C as a source of heat and rejects waste heat to the water 0.100 km below the surface where the temperature is 4.1°C.

Homework Equations

eta = 1 - Tc / Th

The Attempt at a Solution

eta = 1 - 277.1K/292K
eta = -.945548


Not sure what I'm doing wrong, am I not supposed to convert to Kelvin??

 

[JJones_86]

Nevermind, I've figured out what I did wrong, I was treating the equation like this (1-277.1K)/292K