If your triangle isn't right-angled you need to find the area of an irregular triangle and cut it into two pieces.
Start at the corner opposite the longest side.
Go straight towards the long side, making two right-angled triangles.
Now find the area for each one of the little triangles.
Add them together to get the total area of the triangle.
If your area is of an unusual shape, draw a sketch of it and then by using the four shapes above you can break it up into smaller shapes and calculate from there!