Let ah,s be the number of regions formed by h horizontal lines and s other lines as described in the problem. Let Fh,s be the union of the h+s lines and pick any line ℓ. If it intersects the other lines in n (distinct!) points then ℓ is partitioned into n−1 line segments and 2 rays, which delimit regions. Therefore if we remove ℓ the number of regions decreases by exactly n−1+2=n+1.
Then a0,0=1 (no lines means there is only one region), and since every one of the s lines intersects the other s−1 lines, a0,s=a0,s−1+s for s≥0. Summing yields
a0,s=s+(s−1)+⋯+1+a0,0=2s2+s+2.
Each horizontal line only intersects the s non-horizontal lines, so ah,s=ah−1,s+s+1, which implies
ah,s=a0,s+h(s+1)=2s2+s+2+h(s+1).
Our final task is solving
ah,s=1992⟺2s2+s+2+h(s+1)=1992⟺(s+1)(s+2h)=2⋅1991=2⋅11⋅181.
The divisors of 2⋅1991 are 1,2,11,22,181,362,1991,3982. Since s,h>0,2≤s+1<s+2h, so the possibilities for s+1 can only be 2,11 and 22, yielding the following possibilities for (h,s) :
(995,1),(176,10), and (80,21).