Finding $m+n$ from $m+n+mn+1=91$

Question

If $m,n$ are natural numbers such that $m+n+mn+1=91$ .Then how to find $m+n$

Answer 1

$$mn+m+n+1=91\Longrightarrow m(n+1)+(n+1)=91\Longrightarrow (m+1)(n+1)=91$$

But $\,91=7\cdot 13\,$ , so...

Answer 2

Hint: $$m+n+mn+1=(m+1)(n+1).$$