When investigating whether a function is differentiable at a point, there are two things to consider:
1. Differentiability implies continuity. So you need to make sure that d(x) is continuous before you ask if it is differentiable. (Remember, there are 3 conditions of continuity must be explored.)
2. Is the derivative continuous? In other words, does the slope from the right agree with the slope from the left, and agree with the actual slope? Also, are those slopes finite?
Check boundary points x = 0 and x = 16.