Time=distance÷speed. The effective speed with the current is 53+x and against the current is 53-x where x is the speed of the current, so 360/(53+x)=276/(53-x).
360(53-x)=276(53+x), 360×53-360x=276×53+276x, 636x=360×53-276×53=53(360-276)=53×84.
x=53×84/636=84/12=7mph.