Question 718802
Let {{{ s }}} = the speed in mi/hr  of the boat in still water
Let {{{ d }}} = the one-way distance he paddles in miles
Going with the current:
(1) {{{ d = ( s + 6 )*3 }}}
Going against the current:
(2) {{{ d = ( s - 6 )*9 }}}
-------------------
By substitution:
(2) {{{ ( s + 6 )*3 = ( s - 6 )*9 }}}
(2) {{{ 3s + 18 = 9s - 54 }}}
(2) {{{ 6s = 18 + 54 }}}
(2) {{{ 6s = 72 }}}
(2) {{{ s = 12 }}}
The speed of the boat in still water is 12 mi/hr
check answer:
(1) {{{ d = ( 12 + 6 )*3 }}}
(1) {{{ d = 18*3 }}}
(1) {{{ d = 54 }}}
and
(2) {{{ d = ( 12 - 6 )*9 }}}
(2) {{{ d = 6*9 }}}
(2) {{{ d = 54 }}}
OK