You would divide the rectangle into half, to form two congruent triangles. When you multiply the area of 1 of the triangles, it gives you the area of both triangles, and so the area of the triangle as a whole.
this is easy. all you have to do is multiply the denominator (3) by the outside number (12) to get 36. this leaves the equation being x^2 = 36. now you just find the square route of 36 (which is 6).
