blue$)$ as directed.

$$

$($a$)...$using $12$ subintervals of equal width and left endpoints as sample points.

$f{t}^2$

$($b$)...$using $3$ subintervals of equal width and midpoints as sample points.

$f{t}^2$

$($c$)...$using $6$ subintervals of equal width and midpoints as sample points.

$f{t}^2$

$A$~~${}_k={?}^n$