If you're careful about units, then you can do just about anything with
the two given quantities, and the answer will fall out on the desk right
there in front of you:
(2.7 newtons) / (7.8x 10⁵ newtons/coulomb)
Divide numerator
and denominator
by (newton), and
you have (2.7 coulomb) / (7.8 x 10⁵) = 3.46 x 10⁻⁶ coulomb
= 3.46 micro-coulombs
Notice that you didn't describe the direction of the force on the balloon, so
the flavor of the charge on it doesn't matter. A negatively charged balloon
with 3.46 microcoulombs on it would feel exactly the same force, but it would
be in the other direction.
