Given T13 = 15 and T7 = 51.

The general term of an arithmetic sequence is $T_n = a + (n-1)d$.
Given $T_{13} = 15$ and $T_7 = 51$.

Subtract equation (2) from equation (1):

Substitute $d = -6$ into equation (2):

The first term is $a=87$ and the common difference is $d=-6$.
The general term of the sequence is $T_n = 87 + (n-1)(-6) = 87 - 6n + 6 = 93 - 6n$.

1.a)

The term equal to $-21$ is the $\boxed{\text{19th term}}$.

1.b)

Since $n$ is not an integer, $\boxed{\text{66 is not a term of the sequence}}$.

2.

First term $a = 5$.
Last term $L = 61$.
Sum of all terms $S_n = 957$.

The formula for the sum of an arithmetic series is $S_n = \frac{n}{2}(a + L)$.
Substitute the given values into the formula:

The number of terms in the series is $\boxed{29}$.