I was also given a hint:
Represent $C_{42} \cong C_2 \times C_3 \times C_7$, find its group of automorphisms, then look for elements of order 3.
So, I found the group of automorphisms to be:
$ \operatorname{Aut}(C_{42}) = C_1 \times C_2 \times C_6 $
Then I said as 3 divides ($1 \times 2 \times 6 = 12$), we can work out the Euler function of 3, and this gives me:
$ \varphi (3) = 3 - 1 = 2 $
So there are 2 semi direct products. Is this correct?