Well the big hint is that you've been given the position of the origin. Notice that point A is directly above it, so has coordinates (0, 0, 2.5).

We can orient the triangle any way we like, so we may as well have the base along one of the x or y axes. Let's say it's axis x. The bottom triangle extends out horizontally 10m, and then another corner of your shade sail is 2m above that. So the coordinate of point C is (10, 0, 2).

As for the co-ordinate of B, you'll need to use some trigonometry. Since the bottom triangle is equilateral, it can be split into two right-angle triangles by drawing in the perpendicular bisector of the base, to get the height of the triangle. Since the hypotenuse of the RAT is 10m, and the base of the RAT is 5m, by Pythagoras that gives the height of the triangle as . Point B is positioned 3m above it, so the coordinate of B is .

Now that you have the three co-ordinates, I would evaluate the area by evaluating the length of each side of your shade sail and applying Heron's Formula. I'm sure there are other methods too.