Q) A crystal in the shape of a cube is growing in a test tube. Estimate the rate at which the surface area is changing with respect to the side length when the side length of the crystal is 3cm.

This question is from a lesson based on instantaneous rates, rate of change, etc. I don't really understand what to do since I'm kinda new to this unit. Could someone expain the steps on how to solve it so i can solve similar questions on my own?

*answer is 36cm^2/cm

Any help is appreciated.