Let's start b writing down coordinates of all points: A(0,0,0) B(0,5,0) C(3,5,0) D(3,0,0) E(3,0,4) F(0,0,4) G(0,5,4) H(3,5,4)
a.) When we reflect over xz plane x and z coordinates stay same, y coordinate changes to same numerical value but opposite sign. Moving front-back is moving over x-axis, moving left-right is moving over y-axis, moving up-down is moving over z-axis.
It is a measure of how much the function changed per unit, on average, over that interval. It is derived from the slope of the straight line connecting the interval's endpoints on the function's graph.