You inadvertently left out the formula, so I'll do it without the formula first and then again with a formula. We want the length of the red line DB, which is the diagonal of the box. There are two right triangles ABC and DBC. First we calculate the hypotenuse BC of right triangle ABC using the Pythagorean theorem: BC² = AC² + AB² BC² = 8² + 24² BC² = 64 + 576 BC² = 640 BC = √640 BC = 8√10 The required diagonal, the red line DB is the hypotenuse of right triangle DBC. So we use the Pythagorean theorem again DB² = DC² + BC² DB² = 6² + 640 (Notice we only needed BC², not BC) DB² = 36 + 640 DB² = 676 DB = √676 DB = 26 So the diagonal DB is 26 cm in length, without using the formula, but only the Pythagorean theorem. ----------------------------------------------- The formula you omitted is: d = √L² + W² + H² Substituting gives d = √L² + W² + H² d = √24² + 8² + 6² d = √576 + 64 + 36 d = √676 d = 26 cm Edwin