Speed of stream is 4 mph.

A boat travels 6 miles upstream, ande 12 miles downstream.

If you travel downstream, flow of stream helps you and your overall speed increases.

In case of upstream, it resists you.

So, if we consider the speed of boat to be b and speed of stream s,

overall speed ( upstream ) : b - s

overall speed ( downstream ) : b + s

Now,

b + s = 12,

b - s = 6.

So, adding both the equations, 

b + s + b - s = 12 + 6

=> 2b = 18

=> b = 9

So, speed of boat is 9.