/|\
                               / 90\
                              /  |  \
                           a /   |x  \ b
                            /    |    \
                           /   90|90   \
                          /______|______\
                            16      25     

Then you have, by applying the Pythagoras to three different participating right-angled triangles, these three equations


 = ,           (1)

 = ,           (2)

 = .       (3)


By adding equations (1) and (2)  and comparing their sum with equation (3), you get this


 =   ====>

 =  = 2*16*25  ====>

 = 16*25  ====>  x =  = 4*5 = 20.


Answer.  x = 20.