The given series is geometric with common ratio
, which converges if
(i.e. the interval of convergence). We have the well-known result

If you're not familiar with that result, it's easy to reproduce.
Let
be the
-th partial sum of the infinite series,

Multiply both sides by the ratio.

Subtract this from
to eliminate all the powers of the ratio between 0 and
.

Solve for
.

Now as
, the exponential term converges to 0 and we're left with

Answer:
B
Step-by-step explanation:
use the exponent rule of division: you just subtract the exponents if the base is the same, so 4-8=-4
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8 +7m=6m (combine like terms)
8=-m ( subtract 7m on both sides)
m=8 (divide by -1)
5=-1+2n ( subtract 2n on both sides)
6=2n (add 1 on both sides)
3=n (divide by 2 on both sides)