You can probably guess that the area of the tilted rectangle must be 48 because we didn't tell you the dimensions of anything else. Here's how to prove that 48 is the correct answer. The diagonal BC splits the shaded rectangle in half. The upper shaded half is inside the tilted rectangle. Can we show that the unshaded part of the tilted rectangle is equal to half of its area? Imagine a line perpendicular to the tilted rectangle's edge at point A (shown as a dotted line). We see two new rectangles, each divided in half by diagonals (AB and AC). It becomes clear that the unshaded part of the tilted rectangle is equal to half of its area.