So, working out the different side lengths of the triangle is going to help find the area at the end. We split the ratio up into 3s and show them as a proportion of the whole perimeter. For example, one side length is 3/12 of 72, the second is 4/12 of 72 ans the third is 5/12 of 72, meaning the lengths of the sides are 18cm, 24cm and 30cm. Now, to find the area, we can construct this triangle since we are told it is a right angles triangle with the 30cm line being the hypoteneuse (the diagonal), and the 18cm and 24cm lines the base and height of the triangle. So using the formula, Area = (base x height)/2, the area is 216cm^2