2 need some help solving

Problem. 4: The figure on the left shows a vacant lot with a 100-ft frontage in a development. To estimate its area, we introduce a coordinate system so that the x-axis coincides with the edge of the straight road forming the lower boundary of the property, as shown in figure on the right. Then, thinking of the upper boundary of the property as the graph of a continuous function f over the interval [0, 100], we see that the problem is mathematically equivalent to that of finding the area under the graph of f on [0, 100]. To estimate the area of the lot using a Riemann sum, we divide the interval [0, 100] into five equal subintervals of length 20 ft. Then, using surveyor's equipment, we measure the distance from the midpoint of each of these subintervals to the upper boundary of the property. These measurements give the values of f(x) at x = 10, 30, 50, 70, 90. What is the approximate area of the lot? 80 90 110 120 70 -> X ROAD 10 20 30 40 50 60 70 80 90 100 Problem. 5 : Find the area of the region under the graph of f(x) = 4 -Va on the interval [-8, -1]