I don't know how to do it?

a = 130°

Step-by-step explanation:

The interior angle of a polygon is calculated as

180° - exterior angle, thus

180° - 75° = 105°

180° - 42° = 138°

180° - 65° = 115°

180° - 90° = 90°

180° - 38° = 142°

The sum of the interior angles of a polygon is

sum = 180° (n - 2) ← n is the number of sides

Here n = 6, thus

sum = 180° × 4 = 720°

Sum the interior angles and equate to 720

105 + 138 + 115 + 90 + 142 + a = 720, that is

590 + a = 720 ( subtract 590 from both sides )

a = 130

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step-by-step explanation: