To find the missing length you must break this shape into two other shapes. A square, and a right triangle. To do this you take the top value and subtract the bottom. 8-5=3. So one value of the triangle is 3, and the side is 5 since it is parallel and equal to the sides of the square. To find the missing value, you will use the formula "a^2+b^2=c^2." Side A will be our side value of 5. 5x5=25. Side B will be the value of 3. 3x3=9. 25+9=34. And now we find the square root of 34. This is 5.830951895.