Answer
The speed of one rocket relative to the other is $~~0.96~c~~$
Work Step by Step
Let $u'$ be Rocket A's velocity relative to the earth reference frame.
Let $v$ be the earth's velocity relative to Rocket B.
Let $u$ be Rocket A's velocity relative to Rocket B.
We can find $u$:
$u = \frac{u'+v}{1+\frac{u'v}{c^2}}$
$u = \frac{0.75c+0.75c}{1+\frac{(0.75c)(0.75c)}{c^2}}$
$u = \frac{1.5c}{1+0.5625}$
$u = 0.96~c$
The speed of one rocket relative to the other is $~~0.96~c~~$
