Efficiency and Temperature in Heat Engine Cycles: Approaching Parts B, C, and D

[singularcell]

Homework Statement

Need help with b,c,d.

Relevant Equations

Wnet/QH
e-carnot = 1-(Tc/Th)

I didn't have much trouble with part a but I'm struggling with b,c, and d. I considered the efficiency formula for a heat engine e = work done by engine/ qh
but i am unsure of how to approach it.
for part c) not sure how i can get to Tc without knowing Th
for d) my gut is telling me 5/2 but i have no explanation for that. Any guidance/help is appreciated

 

[Chestermiller]

Let's see what you've done so far in part (b).