we have a data set of 15 9 3 -6
firstly we need to identify the first term and the common difference of the data set
the first term is the first number that appears first on the data set
first term = 15
to get our common difference
common difference = second number - first number
9 - 15 = -6
3 - 9 = -6
Tn = a + (n - 1)d
a = 15
n = 10
d = -6
T10 = 15 + (10-1) x -6
t10 = 15 + 9 x -6
T10 = 15 - 54
T10 = -39
the answer is -39