First, we consider a cube of side length equal to x (measured in inches) and surface area equal to A (measured in square inches). We can think of A as a function of x, writing A(x).If the side length x changes over time, then we can think of it as a function of time t (measured in hours) and, as a consequence, we can regard A as a function of time, writing A(x(t)). The chain rule tells us how to compute the derivative of A(x(t)) with respect to t. What's the expression for this rate of change guaranteed by the chain rule?