The slope of the line on a graph of temperature over time for the black can would represent the rate at which the temperature of the can was increasing, while the slope of the line for the silver can would represent the rate at which the temperature of the can was increasing.
Since the black can absorbed more radiation than the silver can, it would have a steeper slope than the silver can, indicating a higher rate of temperature increase over time. This is because the black can absorbed more heat energy from the radiation than the silver can, causing its temperature to increase more rapidly. Therefore, the slope of the line for the black can would tell us that it was warming up more quickly than the silver can, as a result of absorbing more radiation.