\[\boxed{\text{1156.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ x = 5,\ y = 7:\]
\[ax - 2y = 1\]
\[5a - 14 = 1\]
\[5a = 15\]
\[a = 3.\]
\[\textbf{б)}\ x = - 3,\ y = 8:\]
\[5x + by = 17\]
\[5 \cdot ( - 3) + 8b = 17\]
\[8b = 32\]
\[b = 4.\]