Answer
$\frac{1}{3}$a+$\frac{1}{2}$b
Work Step by Step
NM=NB+BM
NB=a-($\frac{2}{3}$a)= $\frac{1}{3}$a
BM=$\frac{1}{2}$b
NM=$\frac{1}{3}$a+$\frac{1}{2}$b
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