Answer
The recessional speed an observer on Galaxy A would find for Galaxy B is $~~0.62~c$
Work Step by Step
Let $u'$ be the speed of Galaxy B relative to Earth
Then $u' = 0.35c$
Let $v$ be the speed of Earth relative to Galaxy A
Then $v = 0.35c$
We can find $u$:
$u = \frac{u'+v}{1+u'~v/c^2}$
$u = \frac{0.35c+0.35c}{1+(0.35c)(0.35c)/c^2}$
$u = 0.62c$
The recessional speed an observer on Galaxy A would find for Galaxy B is $~~0.62~c$
