How do i find the area of a curved Rhombus?

EDIT: it may not even be a rhombus i'm thinking of! :O I mean the shape that looks like a rectangle but the base is longer than the top and the sides have to slant in order to complete the shape. (please give me the correct name for this shape)

Think of it like a toy car race track, the corners of the race track are like curved rectangles, except in this case more like a curved rhombus, so the bottom part is curving in, and that top part is curving out... If that makes any sense.

If it doesn;t make sense for god sake TELL ME! and i will change what i've written, because i really need that answer for this.

what is the formula for this, and if it's complicated, how would i go about working it out, setting it out, ect.