RSA Encryption problem for Discrete Math I am doing practice problems for my upcoming final exam, and am having trouble with this RSA encryption problem. If any one could check to see if i did these correctly, it would be greatly appreciated.

For (a) I got $\mathbb{Z}^*_8 = \{1,2,3,4,5,6,7\}$, 

(b) i got $3*3 = 1 \text{ mod } 8 = 3^{-1} = 3$ 

(c) this one i am not sure how to do

(d) $M = N^d \text{ mod } 15$, i got $d = 3$, so $M = 4^3 \text{ mod } 15 = 4$
(a) $\mathbb{Z}_8^* = \{1,3,5,7\}$.

(b) $x = 3$.

(c) Assuming $a \in \mathbb{Z}^*_{15}$: $a^8 \bmod 15 = 1$ and $a^9 \bmod 15 = a$.

(d) $M = 4$.