In double hashing scheme, the probe sequence is determined by $(h1(k) + ih2(k))\mod m$, where $i$ denotes the index in probe sequence and $m$ denotes the hash table size.
Given $h1(k)$ and $h2(k)$, we have to determine the second element of the probe sequence (i.e. $i = 1$) for the key $k = 90$.
$(h1(90) + 1*h2(90))\mod 23 = (21 + 15)\mod 23 = 36\mod 23 = 13$