Answer:
the Pythagorean theorem was used
Explanation:
You want to know the missing side in a right triangle with hypotenuse 15 cm and one side 9 cm.
Pythagorean theorem
The Pythagorean theorem tells you the relationship between sides 'a' and 'b' and hypotenuse 'c' in a right triangle is ...
a² +b² = c²
9² +?² = 15² . . . . use the given values
?² = 225 -81 = 144 . . . . subtract 9²
? = √144 = 12 . . . . . . . take the square root
The length of the missing side is 12 cm.
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Additional comment
The little green square in the upper corner tells you that is a right angle, and the triangle is a right triangle.
All units of length are in cm.