Question 693460
To find time you need to divide the distance by the relative speed.

Going upstream the relative speed is the speed of the boat minus the speed of the current. Going downstream the relative speed is the sum of the speed of the boat and the speed of the current.

{{{t=d[up]/(v[boat]-v[current])}}}

{{{t=d[down]/(v[boat]+v[current])}}}

Put them together:

{{{d[up]/(v[boat]-v[current])=d[down]/(v[boat]+v[current])}}}

Replace with your numbers:

{{{300/(v[boat]-7)=350/(v[boat]+7)}}}

{{{350(v[boat]-7)=300(v[boat]+7)}}}

{{{350v[boat]-2450=300v[boat]+2100}}}

{{{50v[boat]=4550}}}

{{{v[boat]=91mph}}}