homework and exercises - Gravitation and equilibrium (...and similar problems)

One problem that I'm having trouble with (as opposed to the other):

The Messenger is a probe that orbits Mercury $700 \rm km$ from the surface. What is the tangential velocity it should be rotating at so that it doesn't precipitate towards the planet, in $\rm m/s$?

Data: Mercury's mass $3,3 \times 10 ^{23} \rm kg$ Diameter: $4870 \rm km $ . Gravitational constant: $G=6,67 \times 10^{-11} \rm m^{3}kg^{-1}s^{-2}$

I assume I need to use the following $$a_{c}=\frac {V^2} r$$

$$F=G\frac{m_1 m_2}{d^2}$$

EDIT:

Solving for $V$ in $$G\frac{{{m_1}{m_2}}}{{{d^2}}} = {m_1}\frac{{{V^2}}}{d}$$ did it.