Riangular based pyramid ABCD is shown the base of ABC is a right angled triangle and the pyramid has a volume of 72 cm^3 Dis directly above A AB=AC AD=2xAB

The triangle inequality theorem states that the sum of any two sides of a triangle must be greater than the third side. using this, 

let x = the third side length

4 + x > 15
x > 11

4 + 15 > x
x < 19

x + 15 > 4
x > -11

the third side must meet all of these requirements so it can be any value between 11 and 19.