Answer
The speed of $Q_2$ as measured by an observer in the reference frame of $Q_1$ is $~~0.59~c$
Work Step by Step
Let $u'$ be the velocity of $Q_2$ in our reference frame
Then $u' = 0.400c$
Let $v$ be our velocity relative to $Q_1$
Then $v = -0.800c$
We can find $u$ which is the velocity of $Q_2$ as measured by an observer in the reference frame of $Q_1$:
$u = \frac{u'+v}{1+u'~v/c^2}$
$u = \frac{0.400c-0.800c}{1+(0.400c)(-0.800c)/c^2}$
$u = -0.59c$
The speed of $Q_2$ as measured by an observer in the reference frame of $Q_1$ is $~~0.59~c$
