Estimate the area under the graph of f(x) = 2 cos(x) from x = 0 to x = π/2 using four approximating rectangles and right and left endpoints

So hmm notice the picture below

you're simply being asked to, approximate the area, by using 4 rectangles, like in the graph, it doesn't have to be an exact value, just an approximation, based on the width and height of the rectangles, and getting  their area by that

those rectangles are 0.39 radian long, since if you split π/2 in 4 pieces, you get around that much, the last rectangle there is a bit smaller than 0.39.... but they don't have to be 4 rectangles of equal length

so, get the area of the rectangles, based on their width and height, and sum them up