From 11:00PM PDT on Friday, July 1 until 5:00AM PDT on Saturday, July 2, the Shmoop engineering elves will be making tweaks and improvements to the site. That means Shmoop will be unavailable for use during that time. Thanks for your patience!
Remember that "derivative" means "slope." Since a straight line has a constant slope (the slope is the same everywhere), the slope of the line is the derivative of the line. In this case, f(x) = 4x + 2 has a slope of 4, therefore f ' (x) = 4.