why is this in the pre-calc section?

divide [0,4] into 4 equal sub-intervals, we get the intervals:

obviously (i hope) we have here. if you're not sure, remember where [a,b] is the interval we are considering, and n, of course, is the number of sub-intervals

the midpoints of the subintervals respectively are:

so, by the midpoint rule, our estimate for the integral is given by:

i leave the rest to you