f(a bag of skittles)

Today, I realized mid-class my students desperately needed a review of working with functions to get through the limit definition of derivatives.  The picture below is what spontaneously occurred.  I wrote the left half and then had them tell me should be on the right.  They seemed to really enjoy it, and understood what was going on much better when we tried to apply it to the definition of derivatives.

f(x) = 5x + 1