We must this by try and error
let's look for 7a+1=9b+27a+1=9b+2
7xx0+1=1=9xx0+17×0+1=1=9×0+1
7xx1+1=8=9xx0+87×1+1=8=9×0+8
7xx2+1=15=9xx1+67×2+1=15=9×1+6
7xx3+1=22=9xx2+47×3+1=22=9×2+4
7xx4+1=29=9xx3+2->7×4+1=29=9×3+2→ solution
We can determine that solutions will repeat in a cycle of 63 so we can do like this:
63+29=92=11xx8+463+29=92=11×8+4
92+63=155=14xx8+192+63=155=14×8+1
155+63=218=>11xx19+9155+63=218⇒11×19+9
218+63=281=11xx25+6218+63=281=11×25+6
281+63=344=9xx38+2->11xx31+3->281+63=344=9×38+2→11×31+3→ solution
in fact
344=7xx49+1=9xx38+2=11xx31+3344=7×49+1=9×38+2=11×31+3
Other solution can be found by adding (7xx9xx11)n(7×9×11)n
344+693=1037344+693=1037
1037+693=17301037+693=1730
etc.etc.
693n+344693n+344