You can see that two consecutive terms always differ by 20.
In other words, you generate the next term in the sequence by adding 20 to the previous one.
This implies the following structure:
[tex]\begin{array}{l}a_1=8\\a_2=a_1+20\\a_3=a_2+20=(a_1+20)+20 = a_1+2\cdot 20\\\ldots\\a_n = a_1+(n-1)\cdot 20\end{array}[/tex]
In particular, this implies
[tex]a_{24} = a_1+23\cdot 20 = -8+460 = 452[/tex]
