From the question it is given that,

First term a = \[5\]

n^th term = \[-55\]

Common difference d =  \[2\text{ }\text{ }5\text{ }=\text{ }\text{ }3\]

We know that, a_n = a + (n – 1)d

\[\begin{array}{*{35}{l}}

\text{ }55\text{ }=\text{ }5\text{ }+\text{ }\left( n\text{ }\text{ }1 \right)\left( -3 \right)  \\

-55\text{ }\text{ }5\text{ }=\text{ }\text{ }3n\text{ }+\text{ }3  \\

-60\text{ }\text{ }3\text{ }=\text{ }-3n  \\

-63\text{ }=\text{ }-3n  \\

n\text{ }=\text{ }-63/-3  \\

n\text{ }=\text{ }21  \\

\end{array}\]

Therefore, \[-55\text{ }is\text{ }the\text{ }{{21}^{st}}~\] term.