Since it is a square, the two sides that will form the right triangle with the diagonal is equal. Use the Pythagorean theorem. Hence, x² + x² = 18² , x = ± 9√2, and since it is a square it can't be a negative number, so x= 9√2 milimetres
The question is asking for perimeter, and a square has 4 equal sides. 9√2 × 4 = 36√2 milimetres