Calculate the area of a rhombus of side 250mm and longer diagonal 400mm

Step-by-step explanation:

Refer to the imagine attached.

First of all, let's remember that in a rhombus diagonals are perpendicular to each other, and cut themselves in half. That allows us to focus on 1/4 of the figure (they gray shaded right triangle in my image.

By aplplying pythagorean theorem (or recognizing a 3-4-5 pythagorean triplet scaled up) you can compute the missing side as long, making the shorter diagonal long.

At this point you apply the rhombus area formula, with D and d being the two diagonals

(a bit less than a A4 sheet of paper)