Answer:
-1
Step-by-step explanation:

The formula for area of triangle is -

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So, the area of first triangle =



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Area of second triangle =



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Area of second triangle - Area of first triangle



You would multiply the orignial price by (10%) converted to decimal to find the discount:
21 * 0.10
=2.10
Then, you subract the discount from the original price to see how much he paid:
21 - 2.10 = $18.90