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90 degrees

Step-by-step explanation:

general Pythagoras

c² = a² + b² - 2ab×cos(ABC)

with c being the side opposite of the angle ABC.

c = AC

a = AB

b = BC

(0-15)² + (0-0)² = (0-9.6)² + (0-7.2)² + (9.6-15)² + (7.2-0)² - 2×sqrt((0-9.6)²+(0-7.2)²)×sqrt((9.6-15)²+(7.2-0)²)×cos(ABC)

225 = 92.16 + 51.84 + 29.16 + 51.84 - 2×12×9×cos(ABC)

225 = 144 + 81 - 216×cos(ABC)

225 = 225 - 216×cos(ABC)

0 = cos(ABC)

ABC = 90 degrees