Elias was a poor black worker in south Africa. The time ____he first met Nelson Mandel was a period of his life .he was twelve years old at that time. Nelson Mandel was a black lawyer to____ he turned for advice .Nelson offered____(guide) to poor black people on their____(illegal) problems. Elias needed Mandel's help____he had little education. The school ____he studied for only two years was three kilometers away .he had to drop out of school because his family couldn't pay the school fees____the bus fare .He couldn't read or write well .After trying hard, he got a job in a gold mine ,____ he didn't have passbook to live in Johannesbug and he worried about ____he would become out____work.

已知代数式$\frac{1}{5}$x^a-1y³与-5x^-by^2a+b是同类项，则a与b的值分别是（　　）