Answer
$$\frac{dS}{dt} =624cm^2/s$$
Work Step by Step
Step-1: We know that the surface area of the cube is $S=6l^2$. Let us differentiate this equation,
$$\frac{dS}{dt} = 12l\frac{dl}{dt}$$
Step-2: When $\frac{dl}{dt} = 8$ cm/s and $l=6.5$ cm,
$$\frac{dS}{dt} = 12\times 6.5\times 8=624cm^2/s$$
