Answer:
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
