To answer the question as written: If the length of a rectangle is given as being 50 feet, then the length of the rectangle is 50 feet. To answer the question that is really being asked, if the width of a rectangle is 12.4 feet and the length of 50 feet, the area can be found by multiplying the length by the width. Therefore, the are of the rectangle would be 620 square feet. If however, the question was to determine the perimeter, you would use the formula 2l+2w=p so you would add 12.4 +12.4+50+50, for a perimeter of 124.8 feet.
27 cubic feet = 1 cubic yard.15 inches = 1.25 ftWhat you have is a concrete wall surrounding the base area. If its 4 sides were all lined up straight, theywould stretch (26 + 30 + 26 + 30) = 112 feet, end to end. That's the distance around the foundation.So you have to build this slab that's 112-ft long, 6-ft high, and 1.25-ft thick.Its volume is (112 x 6 x 1.25) = 840 cubic ft.840 cubic ft = (840 / 27) = 31.111 cubic yards(rounded, repeating). Actually [31 and 1/9th] yd3