The equation to find the nth term is:
an = a+(n-1)d
Where a is the first term in the sequence, which is -1 and d is the difference, which in this sequence the next term is found by subtracting 3 from the previous term, so d = -3 and n is the term you want to find.
The equation becomes:
an = -1 +(n-1)(-3)
a24 = -1 + (24-1)(-3) = -1 + (23(-3)) = -1-69 = -70