How to find voltage across resistors in RC Circuit with 2 voltage sources?
Ignore the capacitor to calculate the output voltage just before the switch opens. 

Ignore the capacitor to calculate the output voltage a very long time after the switch closes. 

It goes from one voltage to the other with a time constant of 1.5 seconds- so you know there's a term \$e^{-t/\tau}\$ in the answer.