There is no common factor between 15, 17 and 18 because 17 is prime.
15z² +17z - 18 =
d = 17² - 4.15.-18
d = 289 + 1080
d = 1369
z = (-17 +/- \/1369) : 2 * 15
z = (-17 + 37) : 30
z' = 20 : 30 = 2/3
z" = (-17 - 37) : 30
z" = -54 : 30 = -1.8
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