Step 1: Use orbital velocity relation.
For circular orbit:
\[
v^2 = g' r
\]
where \(g' = 8.4 \, \text{m/s}^2\).
Step 2: Convert velocity.
\[
v = 8.4 \, \text{km/s} = 8400 \, \text{m/s}
\]
Step 3: Find orbital radius.
\[
r = \frac{v^2}{g'} = \frac{(8400)^2}{8.4}
\]
\[
= \frac{70560000}{8.4} = 8400000 \, \text{m}
\]
\[
r = 8400 \, \text{km}
\]
Step 4: Find height above Earth.
\[
h = r - R
\]
\[
h = 8400 - 6400 = 2000 \, \text{km}
\]
Step 5: Final conclusion.
\[
\boxed{2000 \, \text{km}}
\]