Affine Cipher: what to do if the number isnt invertible over mod n?
If you divide the whole equation throughly 2 you get
$$31a\equiv 19\pmod {34}$$
which you can solve to get $a\equiv 5\pmod{34}$. This then gives two possibilities for $a\pmod {68}$: either 5 or 39. For these two possibilities you can then find the corresponding $b$ (it turns out to be 5 in both cases) and check which solution works.