Answer
$a = 42$ km
Work Step by Step
1. Find the angle $A$
$A + 40 + 70 = 180$
$A + 110 = 180$
$A = 70˚$
2. Solve for $a$
$\frac{a}{sin(70)} = \frac{42}{sin(70)}$
$a = \frac{42sin(70)}{sin(70)}$
$a = 42$ km
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