The triangle's height must be 3 times that of the rectangle for the areas to be the same.
For a triangle, A = (1/3)bh
For a rectangle, A = bh
If we let h represent the height of the rectangle and H represent the height of the triangle, we have
.. (1/3)bH = A = bh
.. H = 3h . . . . . . multiply the above equation by 3/b