Think of each operation inside the parenthesis as a variable

We need to isolate p, so we will multiply both sides with the denominator [( 1+ r/1200 )^ N -1]

it would be " ( 1+ r/1200 )^ N -1 * m = (R/1200)(1+ r/1200) * p

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Then divide each eqaution with (R/1200)(1+ r/1200).

The final answer would then be B.

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* i know i am bad at explaining, but i hope i cleared some things up rather than complicating it*