Not exactly sure what's being asked, but I'll assume this curved wall is cylindrical (i.e. the curve is a section of a circle and the height is a fixed vertical length). Let's pick a center point for the circle; draw a radius "R" to each end of the 168" line connecting the two ends of the arc. The result is obviously an isoceles (spelling?) triangle. Now add another radius "R" bisecting the angle at the center point as well as the 168" line. Of course, the intersection forms right angles, and the length of this line from center to line is "R" minus the 40". This means that R*R = 84*84 + (R-40)*(R-40), so you should be able to determine R (I get 432.8, but I always make math mistakes). Knowing all three sides of the isoceles triangle, you can find the angle between the two "R" sides. That angle divided by 360 and then multiplied by 2*PI*R should give the length of the arc. Multiply by the height and you should be done.