X-ray pulses from Cygnus X-1, a celestial x-

Question

ray source, have been recorded during high-
altitude rocket flights. The signals can be
interpreted as originating when a blob of ion-
ized matter orbits a black hole with a period
of 7.84 ms.
If the blob were in a circular orbit about
a black hole whose mass is 13.5 times the
mass of the Sun, what is the orbit radius?
The value of the gravitational constant is
6.67259 × 10−11 N · m2/kg2 and mass of the
Sun is 1.991 × 1030 kg.
Answer in units of km.

Answer 1

Use the same formula derived in my answer to your Saturn moon question, but this time solve for R.