In order to determine the value of the missing length b, use the Pithagorean theorem, given by:
h² = a² + b²
where:
h = 6.5 mm
a = 2.5 mm
solve the previous equation for b, as follow:
b = √(h² - a²)
b = √(6.5² - 2.5²)mm
b = √36 mm
b = 6 mm
Hence, the value of the missing length is 6 mm