I was given a function here and told to do two things which t is time in hours

$\displaystyle f(x)=20+15e^{kt}$

A.) After 2 hours the persons temperate was 33 C find K

which gave me answer $\displaystyle \frac{ln(\frac{13}{15})}{2}$

B.

Find the rate of change after 4 hours

So what i do is takeout derivative and plug in 4 or solve k for 4 hours and and then take derivative and solve