Sometimes you can say this is derivative of the inside times the derivative of the outside with respect to the inside. So either way, it's negative four e to the negative four x for x is greater than zero. So we did the first part, we expressed f prime of x as a piecewise defined function. We didn't define the derivative, actually I forgot a parenthesis here. We didn't define the derivative when x is equal to zero because it's actually not going to be defined there. Now let's do the second part. Find the value of x for which f prime of x is equal to negative three.