Answer
Kindly see explanation below.
Work Step by Step
To find the are under the graph $f$, firstly we will have to approximate the area by $\bf{Rectangles}$.Approximate the area can be found by sketching four rectangles. The area $R_4$ of this approximation can be found as:
$\bf{R_4= f(x_1)(x_1-a)+ f(x_2)(x_2-x_1)+ f(x_3)(x_3-x_2)+ f(b)(b-x_3)}$
