Hah. Look at my fucked up way of doing it was.

Diagram attatched.

Seeing as we know that the bottom is 3 times longer than the top traingle...

I drew a similar isoceles triangle as shown, used the area ratio thing and got 450cm^2 for that similar traingle. Now seeing as the area of that triangle is

(x^2) / 2 (base=height as its an isocles)

Will I worked it out and I got x=30. Therefore seeing as the 3 became a 30 (*10), the one became a 10. Therefore the area of the rectangle is 300.

Area of trapzeium = area of rectangle + area of triangle.

Area = 450+300 = 750

I know, way too long and useless. It probably wouldnt have worked as well if the smaller triangle wasnt similar to the triangle it was within, so yeah. Im an idiot, haha.