Answer:
13.5 years. 
Step-by-step explanation:
We know that exponential growth function is in form  , where, a is initial value and r is growth rate in decimal form.
, where, a is initial value and r is growth rate in decimal form.
Population of goldfish after x years would be  .
.
We know that a linear function is in form  , where, b is initial value and m is slope.
, where, b is initial value and m is slope.
Population of minnow after x years would be  .
.
Graphing both equations, we will get our required graph as shown in the attachment. 
Since both graphs intersect at  , therefore, in the 13.5 years both populations will approximate the same.
, therefore, in the 13.5 years both populations will approximate the same.