The first term is -43.
Explanation:
an = a + (n - 1)d
a12 = a + 11d = 56, first equation
a21 = a + 20d = 101, second equation
Subtract the second equation from the first equation, cancelling +a and -a
11d - 20d = 56 - 101
-9d = -45
d = 5
Subtitute d into the first equation
a + 99 = 56
a = -43
a refers to the first term, hence the first term is -43.
