Water in the murray flows at 4km/hr. If a boat takes 3 hours to complete 32km round trip (16 km downstream and 16km back upstream), what is the boat's relative speed to the water?

so far i have found that when going upstream, the equation is (x + 4) and when going dfownstream the equation is (x-4) x being the speed of the boat. and the boats speed is 32/3

but i am unsure how to turn this into an algebratic equation.

thanks for your help

Zac