一道英文物理题A rock falls through water with a continuously decrea

一道英文物理题
A rock falls through water with a continuously decreasing acceleration.Assume that the rock's acceleration as a function of velocity has the form ay = g - bvy where b is a positive constant.(The +y direction is directly downward.)Prove mathematically that if the rock is released from rest at time t = 0,the acceleration will depend exponentially on time according to ay(t) = ge-bt.
要翻译的话说下，
最后是个指数函数ay(t) = ge^-bt，上标没了。所有的y都是位于a的下方表示方向，没有实际意义.