Question 858907
Lookup of that question shows that somebody answered it in that question posting.  Be aware, some of the given values there are different from the ones you used in your posting here.


Let c = the speed of the river current.
R*T=D for rate, time, distance, basic uniform travel rate relationship.


________________speed__________time__________distance
DOWNSTRM._______22+c___________t_____________2.1
UPSTREAM________22-c___________t_____________1.7


The two times are described as being equal, and both are t.
Downstream equation will give {{{t=2.1/(22+c)}}}; and the upstream equation will give {{{t=1.7/(22-c)}}}.


The two expressions for time being equal,
{{{highlight(2.1/(22+c)=1.7/(22-c))}}}.


{{{1.7(22+c)=2.1(22-c)}}}
{{{1.7*22+1.7c=2.1*22-2.1c}}}
{{{1.7c=2.1*22-2.1c-1.7*22}}}
{{{1.7c+2.1c=2.1*22-1.7*22}}}
{{{3.8c=22(2.1-1.7)}}}
{{{3.8c=22(0.4)}}}
{{{3.8c=8.8}}}
{{{c=8.8/3.8}}}
{{{highlight(c=2.31)}}} or {{{highlight(c=2.32)}}}  ------ this could be done as a rational number result, but the accuracy would not be needed that way.