V is going to give us the volume as a function of time. Volume is in liters, and time is in minutes. And so if they're talking about the slope of the tangent line to the graph, the slope of the tangent line to the graph of v, that's just v prime. So if you take the derivative with respect to time, that's going to give you v prime, and these are all functions of t. These are all functions of t. And they say at t equals seven, it's equal to negative three. So this, which is the same thing as the slope of tangent line slope of tangent, tangent line, and they tell us that v prime of, at time equals seven minutes, our rate of change of volume with respect to time is equal to negative three. And so you could say, if we were to write it out, this means that after, after seven minutes, seven minutes, the tank is being drained at an instantaneous, instantaneous. That's why we need that calculus for that instantaneous rate.