pls explain step by step

In the graph, /(1) = 16. /(2) = 10, /(3) = 7, /(4) = 3. y = fox (2, 12)) (a) Approximate the shaded area under the graph of f by constructing rectangles using the left endpoint of each subinterval, (Use symbolic notation and fractions where needed.) Act = (b) Approximate the shaded area under the graph of f by constructing rectangles using the right endpoint of each subinterval. (Use symbolic notation and fractions where needed.) Aright =