[tex]\bf \begin{array}{lccclll}
&distance&rate&time\\
&-----&-----&-----\\
upstream&3&r-4&t\\
downstrea&11&r+4&t
\end{array}\\\\\\ recall\implies d=rt\\\\
-----------------------------\\\\
\begin{cases}
3=(r-4)(t)
\\\\
11=(r+4)(t)\\
--------------\\
3=(r-4)(t)\to \frac{3}{r-4}=\boxed{t}\qquad thus
\\\\
11=(r+4)(t)\to 11=(r+4)\left( \boxed{\frac{3}{r-4}}\right)
\end{cases}[/tex]
solve for "r"
