I know I'm in middle school but i can help you.The answer is 12 your correct.
For a function to begin to qualify as differentiable, it would need to be continuous, and to that end you would require that 
is such that
Obviously, both limits are 0, so 
is indeed continuous at 
.
Now, for to be differentiable everywhere, its derivative 
must be continuous over its domain. So take the derivative, noting that we can't really say anything about the endpoints of the given intervals:
and at this time, we don't know what's going on at , so we omit that case. We want to be continuous, so we require that
from which it follows that 
.
It is given that AB is parellel to CD. These two lines are cut by a transversal, creating angles BAC and DCA. Thus, angle BAC is congruent to angle DCA because alternate interior angles are congruent. It is also given that angle ACB is congruent to angle CAD. Therefore, triangle ABC is congruent to triangle CDA because of the ASA theorem.
Can you show us the triangular prism to help you
