\[\boxed{\text{584\ (584).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x_{n} = x_{1} + d(n - 1)\ \ \]
\[x_{1} = x_{n} - d(n - 1)\]
\[\textbf{а)}\ x_{30} = 128;d = 4:\]
\[x_{1} = x_{30} - d(30 - 1) =\]
\[= 128 - 4 \cdot 29 = 12.\]
\[\textbf{б)}\ x_{45} = - 208;\ \ d = - 7:\ \]
\[x_{1} = x_{45} - d(45 - 1) =\]
\[= - 208 + 7 \cdot 44 = 100.\]