Section 6.7 Quiz 7.7	Question:	A radioactive substance decays exponentially, so the amount A left at time t is given by A  =  A0 ekt, where k < 0. If there are 500 units of the substance initially and 149 units after 6 hours, what is the decay constant k (in hr-1)? Express your answer to three decimal places and include the negative sign.