linear diophantine equation: ax+by=c where a,b,c are given and range of x and y are given. 1.)how to find the number of integral solution between the range? 2.)find x and y for which x+y is minimum

If we Know the value of c where c=((a%mod)*(b%mod))%mod , the value of (a%mod) is given.How to find the value of (b%mod). a,b,c are integer and mod=1e9+7.

Example: mod=1e9+7 c=386724027 (a%mod)=787871 how to find the value of (b%mod)? ans=9192351 Can anyone help me to solve this?