If the measurements are 8, 11.3, and 8 and the formula for the area of a triangle is 1/2 of base x height, then you would do 1/2 of 8 (or 11.3 depending on which one is the height) x 8.
btw, the measurements that you would use are the ones that make up straight lines of the right triangle. the height would be the straight line going up, and the base would be the straight line going sideways.
hope this helps :)