The value of (428×428×428+348×348×348)/(428×428-428×348+348×348) is:

$\frac{428×428×428+348×348×348}{428×428-428×348+348×348}$

= 

a³ + b³ = ( a + b ) ( a² + b² - ab )

( a + b ) = \(\frac{a^3 + b^3}{a^2 + b^2 - ab }\)

So by comparing we get =

a = 428

b = 384

So, a + b = 428 + 342 = 776